It looks like I'll be doing something similar. For a while, I pondered how expensive foam boards are at Lowe's and Home Depot and eventually found a place (from Craigslist) which produces styrofoam for insulated concrete forms and had a lot of factory seconds to get rid of cheap. Basically, they are 4 ft long, 1 ft high, 2-4 inches thick and fit together like legos. How large is your chamber? Has that door served you well and is it just plywood? That's the other awkward thing which is expensive to purchase, but needs to seal well and not warp over time.