I'm planning to go to Habitat for Humanity Restore and buy an old angle iron bed frame that I'll use to make my platform bed. I'll use slats and I'll build a platform with no wood at all and no welding. It'll be magic -- or a horrible, tragic failure.
It's nearly finished in my head, now all I have to do is make it appear on the bus.
My main concern for my plan is how to separate the cold air in the garage from the warm air in the bus. Plywood would do that and in the absence of a solid barrier to the (relatively) unheated storage area I need to come up with a vapor barrier of some kind. Like TheHubbardBus I don't want to start carrying 4x8' sheets of ply products onto my bus for a couple reasons.