by boardhead » Thu Dec 20, 2007 6:30 pm
If you are building up the floor using 1/2" plywood sheet on top of a frame made from 2x2s, and seal the bottom with asphalt emulsion or similar, don't worry about the gap in the frame cross members. Filling in the gaps just creates additional work, and the side rails provide more than enough support. This is how I built mine, and it's worked out great.
If you plan to just lay the plywood floor on the metal chassis without framing it, you'll need to fill in the gap - unless you like a bouncy bed.