[toolingjim]

We built an enclosure like this a couple of years ago for my father-in-law. We used 2" foamboard insulation, foamboard adhesive, and a few drywall screws. Spend the money on the special adhesive: some standard construction adhesives will melt the foam. One sheet made the whole enclosure. My father-in-law added the finishing touch by making a seal out of strips of old blanket stapled to the floor. Totally eliminated attic draft leakage, and made his small (1000) square foot house much more comfortable.
By the way, the cover on my attic stairs is 1x6 boards with 4" of fiberglass stapled to it. It came with the house. If I ever get around to replacing it I will go the foamboard route with a soft foam seal.