Paint the cubbies and a sheet of 3/4-inch MDF or plywood sized 40-by-80 inches, if desired.
Position the two cubbies back to back on the ground, so that the storage cubbies run either side. The ends should be flush.
Place the sheet of MDF or plywood on top of the cubbies so that the edges of the plywood or MDF sheet match up all around with the edges of the cubby units.
Drill countersunk pilot holes sized slightly smaller than 1 5/8-inch wood screws around the perimeter of the MDF or plywood sheet, and a double line around 1 inch from either side of the center of the plywood or MDF sheet. Drive 1 5/8-inch screws through the pilot holes to secure the sheet down. There should be one screw approximately every 5 to 6 inches.
Position the twin storage bed in place in the bedroom and place a twin mattress on top.