I just built an insulating door for my attic stairs last week. Because of the height of the attic stairs when folded, I could not install a simple flat door. I built a box out of 1x8s and 3/8 inch plywood and insulated it with foam panels. The box is open on the bottom and it is hinged to swing up like a door. I used foam weatherstripping as a gasket. Two thinks you want to do -- (1) stop air leaks, and (2) insulate to minimize the heat loss.
Also, a company in Baltimore makes insulated attic stairs. I think that their name is Resource Conservation Technology. Area code 410. No web site.