Jay-J gave you the info for a company that sells insulating attic stairs and hundreds of types of weatherstripping. Chck it out.
I built a box that works great. The box has two main functions -- insulation and sealing. It may be that the sealing function is more important than the insulating function but you may as well accomplish both.
I built a box from 1/2 inch plywood and caulked all the joints to make it airtight. Then I hinged it so it would swing open into the attic. Cover it with 2 inch foam, glued down with construction adhesive. I covered the sealing edge of the box with high density foam or rubber weatherstripping. Make certain you leave enough room for the weatherstripping when you set the hinges. I used a couple of sash locks to pull the door tight when it is closed, thus making a positive air seal.