Evolving from the buildyourcnc design based on tips from this forum I decided to try and build a torsion box for the bed. It is not quite as complicated as a Joe's build, but provides a relatively flat surface and good strength. It is built from 3/4" MDF, with the bottom skin made of 1/4" plywood.
Another worry I had at the beginning based on other's tips was warpage of the MDF, so I tried to prime and paint along the way and even primed the inside of the torsion box. (My house can suffer some high humidity especially through the winter in the redwoods of California).
Note: These pics were taken on 7/01/2009