fold out couch
It's easy get a sheet of 3/8s inch CDX plywood. $20. Cut the plywood the size of the space under the couch and try it for fit. Get enough 1 X 2 or scrap lumber to make a frame the size of the plywood and one piece across the middle. Get some 1 and one half inch wallboard screws and screw the box together then screw the plywood to the box frame. Slip the plywood into the underside of couch and use the 1 and 1 half inch screws to screw the box frame to the frame of the couch. If there is a hole that you can reach thru to get behind the drawer put a little door in the plywood so you can reach in. Don't us any glue just in case you need to take it out.