I'm in the same situation. However, I have come up with a plan! If you purchase 5 rectangular milk crates (sold on AMAZON for about 5 dollars a piece) arrange them like a 5 dotted dice on the floor, upside down, then lay a sheet of ply wood over the milk crates, that should give you a sturdy base to lay your mattress on! If you feel that it needs to be even sturdier, then you can attach inexpensive pvc piping and connect the outer milk crates together. Leaving the center crate on it's own. The whole project should cost well under $100.