In looking at what the manufacturers do, it seems they just screw stuff to the plywood paneling and call it good. With fairly standard flathead screws. Obviously there are exceptions for big fixtures and such.
Thanks for the tip on the framing. That's a good idea. Typically for bunk beds and dinettes, they get away with 3/8 plywood and little or no extra support at all. But I don't think that will be quite strong enough in this project with the 32" span. I was thinking of just building up the underside of the plywood with some cleats. Anyway, I have enough scraps to try some things and see what works. By the time I get to working on that I will be a happy guy! I want to see the box get done!
:)