Slat-style beds without box springs -- such as bunk beds and certain platform beds -- can be downright uncomfortable when paired with basic foam or other insubstantial or thin mattresses. A bunkie board remedies this discomfort. The fabric-wrapped plywood base provides just a little height and, more importantly, a bit more structure or a flat surface to eliminate the bumpy feel transferred by the slats. Also, the board reduces wear on the mattress caused by friction or rubbing that otherwise occurs between it and exposed slats.
Remove the mattress. Measure the bed frame area between the side and end rails occupied by the mattress. Subtract up to 2 inches from the width and length -- in other words, an inch from all four sides. This will be the bunkie board size; adjust it accordingly -- the shortened size allows room for the wraparound padded fabric that softens the bunkie board's edges and surface, making them less likely to puncture or damage the mattress. So, the thicker the padding, the smaller the board.
Cut the 3/4-inch plywood to the determined size, using a circular saw and carbide blade designed for plywood to avoid splintering. If your twin-sized bed or twin bunk-bed frames each measured 38 by 75 inches, for example, you may want to cut the board to 36.5 by 73.5 inches, allowing an inch and a half on each span for moderately thick upholstery-grade quilted fabric.
Abrade the board's edges and any roughness on the top and bottom sides, using medium-grit sandpaper. A power sander makes relatively quick, easy work of a large plywood-sanding project. If the wood is at least somewhat smooth, it's less likely to cause wear or come through the material over time.
Spread out the quilted upholstery smoothly on the floor or other large work surface, wrong side up, if applicable. Cut two pieces -- one for the top and one for the bottom of the bunkie board, using scissors. Although the bottom one can measure the board's exact width and length, the top piece should be a few inches longer on each side to wrap around the plywood, overlapping the fabric on the bottom for stapling.
As an example, if you're making a full-sized bunkie board at 53 by 74 inches, cut the bottom fabric to this size, and the top piece to about 59 by 80 inches, leaving a few inches on each side for stretching around.
Spray the plywood's upward-facing side with fabric adhesive. Smooth the exact-fitting bottom upholstery or dust-cover over the board, centering it lengthwise and widthwise. Flip the plywood over. Spray the board's now-upward facing side with adhesive. Lay the slightly over-sized piece of fabric smoothly into place, centering as with the other side, and folding and stapling each corner.
Fold and staple the excess quilting onto the board rather than trimming it, to secure the overlapped edges and corners, and for a more padded finish.