Here is what I did with the unused space under our sofa sleeper.
I removed the front kick board from the sofa and attached a piece of 2 x 4 with a 24" full extension drawer glide to the metal frame for the sofa on each side.
I then built a box out of 1/2" pine plywood measuring roughly 48" wide x 32" deep x 8" high.
I then took the metal angle brackets that attached the kick board to the sofa and used them to connect the kick board to the front of the drawer box. Sorry these picks are a little dark. But here is the finished project.