Re: skirting using foam and wood
Well, there are several methods. The cleanest way would to cut to size and lay the 2x4s in the right shape for each section of the frame, considering the area to be covered and the size of the foam (for minimal cutting and waste). Then connect them together using glue and long screws from the outside edge, flat connectors (screw or nail) from the back, or a triangular block or metal 'L' and short screws from the inside corner. Cover with thin plywood/plastic/aluminum (attached with nails or staples) to protect the foam. Then install the foam, holding it in place with nails or staples. If you need to dissassemble it for storage or transport, don't use glue, and use phillips screws, not nails.
To hold the sections together, I'd use metal strips and screws. or perhaps eyes and bungie cords or wire, or hinge and latch, if I needed access.