I keep thinking that a better homemade approach to a hot box would be to use lighter materials with high R-values like this:
If you build some sort of collapsable rack and frame (think tents) and use foil faced insulations you could easily achieve the same thermal characteristics as a rigid foam or batt insulated heavy, immovable plywood ski coffin. It could be more portable and easier to store in the off season. You could even bring it on trips.
What about old down or synthetic sleeping bags??
You're trying to build a enclosed thermal volume (oven) where skis can hangout for a while......not a house.
Another option is consider forgoing the plywood and just use the rigid insulation attached to a frame. Polyisocyante is a higher R-value insulator than the polystyrene per inch.