Yep, got that all figured out. The sides will be hinged directly to the bottom. The ends will be hinged to 3/4"x3/4"x bed width riser boards that will be glued/screwed to the base. Add a few simple latches at the corners and it will work great. Fold the sides in, fold the ends down over the sides and you have a 2 1/4" thick platform. I like the 3/4" thickness since plywood is so crappy nowadays, any thinner than that and it would not hold its shape. Also, that thickness will give added protection to the valuable sheet metal. A little heavy but you aren't hauling it very far. Give a buddy a Bud to help move it in and out.
The liner is strictly functional, I don't care how it looks so plain old plywood, glue the carpet on. At the shows, fold it up and lay it with the carpet looking up and use it to set your hibachi and lawn chairs on so they don't sink into the grass. Or, reassemble it upside-down and use it as a tall coffee table. That would be more practical than trying to get PVC pipe to hold it up and you won't need to carry around extra junk. Anyway, we are supposed to be roughing it at rod runs so bending over to grab a Bud off a short table is a badge of courage.