my father had a bed built about 25 years ago. it was a nice foundation bed made out wood. to be honest the whole thing was made out of decking or something (they were very poor). anyway, the carpenter made it so that the bed had drawers underneath which turned out to be very practical but he did use 3/4 plywood to throw the mattress on top of. he made the base of the bed high to allow for regular space under the bed for the drawers and also to raise the bed to simulate that there was a box spring under the mattress.
this thing is a monster, its probably the most basic concept of a bed i've ever seen. its a rectangle underneath, the frame. there are drawer slides for the drawers. then the footboard, rails and headboard are attached using giant 1/2 inch thick 6 inch long lag screws. 2 of them at each end so 4 screws per side.
and then the bed rails are screwed down to the box frame and then you throw 2 pieces of plywood down on top of that.
its perfect. the mattress is still in awesomecondition, no sags or anything and the bed while not the prettiest thing, still has the same mattress, same plywood and everything is still good after 8 different moves.
i guess the point is, just get 3/4 plywood. it hasn't failed us yet.