c/d:boxsprings

if you have a decent frame (with stable support in the centre) you can get a plank of chip board cut at the local home depot (or whatever you have there) that should support a mattress fine. You can also buy half box springs - or saw the thing yourself!

I sawed one in half once and then screwed it back together. Bad idea. Sagged a lot. Use plywood on top of some 2x4s (like 5 or six of em, running across the bed frame.) Just plain plywood will sag after awhile.