How to Build Modular Rehearsal Cubes for Small Black-Box Theatres

In a black box, the set is rarely finished. It is a suggestion — a level, a step, a sudden change in height that makes an entrance mean something. Rehearsal cubes do that work long before the design is locked: three of them become a bench, a windowsill, a grave, a balcony, a shop counter. The cubes that earn their keep are light enough for one person to carry, rigid enough to be jumped on, and simple enough to rebuild when the plywood finally gives out. What follows is how to make a working family of them without a workshop full of machines.

Decide on a Small Family of Sizes

Resist the urge to build one size. Six identical cubes are less useful than four cubes, two half-cubes and one long bench. The trick is to standardise the footprint and vary the height, so any combination stacks into levels you can actually use.

Pick a base module. In a small black box, 600 mm (about 24 in) square is a good compromise: it passes through a standard doorway, two people can carry a stack of them, and it reads as a stool, a step or a low table depending on how it is oriented. From there, a workable kit looks like this:

• Four full cubes at 600 × 600 × 600 mm — the workhorses, for sitting, standing and stacking.
• Two half-cubes at 600 × 600 × 300 mm — steps, footstools, the thing an actor perches on to look casual.
• One bench at 1200 × 600 × 600 mm — doubles as a table, a bed end or a hiding place for props.

Keeping one dimension constant matters more than it sounds. A 600 cube plus a 300 half-cube gives you 900 — roughly counter height. Two cubes gives 1200, close to shoulder height on many performers. Matching real furniture heights (a chair seat sits near 450 mm, a dining table near 750 mm) means blocking on the cubes translates honestly to the final set. Write the numbers on the side of each cube in marker; nobody remembers at eleven at night.

Choose Materials That Stay Light

Weight is the whole game. A cube that two people grunt over will not get moved during a rehearsal, and a cube that never moves is just a badly placed platform.

Use 12 mm plywood for the panels and softwood for the internal structure. Birch ply is strong and takes paint well; poplar ply is lighter and cheaper and perfectly adequate for rehearsal stock. Avoid MDF, which is heavy, hates damp and strips screw threads the first time a cube is dropped. If you can only get 18 mm ply, use it for tops and bottoms and keep the sides thinner.

What makes a thin panel behave like a thick one is the frame inside it: 44 × 44 mm softwood cleats glued and screwed around the inside of the top and bottom, plus a vertical corner block in each of the four corners. Those corner blocks do double duty later as anchors for bolting cubes together. Use a decent wood glue and plated screws long enough to bite without breaking out the far face — pilot holes and a countersink bit will save you more time than any power tool in the shop.

Build the Box

The construction is a tube with a lid and a floor. For a 600 mm cube in 12 mm ply, cut the top and bottom panels at 600 × 600 mm and the four side panels at 576 × 576 mm. The sides sit inside the top and bottom, which gives you a small overhang that protects the side edges and makes stacking forgiving.

1. Cut and check square. Cut all six panels and the cleats. Measure the diagonals of every panel — if the two diagonals differ by more than a couple of millimetres, trim or recut. Out-of-square panels are the reason cubes wobble.
2. Make an L. Glue and screw a corner block to two side panels, driving screws from the outside through the ply into the block. Stand the L on a flat floor and check it with a try square before the glue grabs.
3. Close the tube. Add the third and fourth sides the same way until you have an open-ended box. Check both diagonals of the open top; adjust with a ratchet strap before the glue sets.
4. Fit the cleats. Glue and screw cleats around the inside of the top and bottom panels, set in from the edges so they sit inside the tube.
5. Floor first, then lid. Screw and glue the bottom panel on, check square again, then attach the top. Working bottom-up means the last operation is the one you can see and correct.
6. Fill and sand. Fill the screw holes, sand the arrises to a 3 mm round, and knock back any glue squeeze-out before paint.

One person can put together a cube in an afternoon this way. Build a jig — a simple plywood corner frame that holds two panels at exactly 90 degrees — and the second cube takes half the time of the first.

Join Them Without Wobble

Loose stacking is fine for a read-through. For anything with movement, bolts are better.

• Bolts and threaded inserts. Fit M8 tee nuts or threaded inserts into the corner blocks, and drill matching clearance holes in the faces that will join. Drill them with a jig so the pattern is identical on every cube — this is the single biggest time-saver in the whole build.
• Alignment pins. Short lengths of 12 mm dowel in the top face and matching holes in the bottom stop a stack from skating while you line up the bolts.
• Straps for speed. During a rehearsal you often need a change in ten seconds. A cam strap around a stack is faster than four bolts and honest enough for blocking.
• Brace the tall stuff. Anything stacked three high should be braced to something, or at least splayed at the base. Bolts hold cubes together; they do not stop a tower from tipping.

Paint the hardware matte black. Nothing kills a black-box moment like a shiny bolt head catching a sidelight.

Edges, Faces and Other Small Decisions

Finish the cubes as set, not as furniture. Two thin coats of matte black, rolled on and sanded lightly between coats, will outlast one thick coat and touch up far more easily. Gloss is a mistake: it reflects, it scuffs white and it never matches. Leave a couple of faces unpainted or coat them in chalkboard paint — actors will write blocking notes on them, and pencil on bare ply is easier to erase than marker on paint.

Fix rubber or felt pads to one designated face of each cube so it grips the floor and does not scratch it. Mark that face clearly. Cut 100 mm hand holes in two opposite sides of the full cubes, with the corners rounded — they make lifting genuinely easier, and the loss of stiffness in a panel that is already braced is negligible. Do not cut them in half-cubes, where there is not enough material left to keep the box rigid.

Test, Label, Store

Before a cube goes into a rehearsal, stack the tallest arrangement you expect to use, put your weight on it, and shake it. Creaks are information. Any cube that flexes visibly, or whose screws have started to open up their holes, is repaired or retired — do not let it become the one that fails in performance.

Then label it. Size, date of build, and a simple load note such as “one person standing” or “seated only”. Inside the lid, a pencil line records the last inspection date. Check every screw and bolt before each production; a two-minute pass with a screwdriver prevents most of the surprises.

Store cubes in size order, heaviest at the bottom, and never more than three high without a wall or rack behind them. Keep them off damp concrete and out of the damp corner of the store, because plywood delaminates quietly and then all at once. When a panel does fail, replace the panel rather than patching it — the fix costs an hour, the failure costs a get-in.

The Pre-Show Check

Five minutes, every night, before the half: walk the cubes, press each one with the flat of your foot, look for splits at screw holes and check that the bolts you can reach are tight. Move anything that has crept. If you plan to stack higher than two cubes, or to put people on top of a stack under load, get a structural engineer to look at the arrangement — no set of steps on a page can certify a structure, and the answer is usually a small brace that takes ten minutes to fit. Build the family once, check it often, and the cubes will quietly do their job for years.

Photo: crazy motions / Pexels