Make your own bunkie board with 3/4-inch plywood and quilted fabric. Cut the board to 2 inches smaller than the frame of the bed in both directions and use spray adhesive to hold it in place. Finish the edges using a staple gun to fasten the fabric in place. Bunkie boards make foam mattresses more comfortable on futons, slatted platform beds and other beds where a box spring is not appropriate.
Begin the process by measuring the bed. A single sheet of plywood is sufficient for most twin beds, but full and larger beds require two sheets and separate bunkie boards for each side. Subtract the 2 inches from each dimension before cutting the plywood. Use a skill saw fitted with a plywood cutting blade to make the cuts and prevent splintering of the wood. If you prefer to avoid making cuts yourself, most home improvement centers offer to cut sheets you purchase in the store for a nominal cost. Use sandpaper to smooth the cuts and any rough spots on the surface.
Cut a piece of thin fabric that complements the quilted cover to fit the bottom of the board. Cover the board with spray adhesive and attach the bottom cover to the board. Cut the quilted fabric so it is 6 inches longer than the board in both directions. Lay the fabric flat on the floor, spray the uncovered side of the board with adhesive and lay it centered on the fabric. Fold the fabric over the edges and attach it with staples.