BR submitted this note and sketch on using a piece of rigid foam board to seal and insulate an attic pull-down stairway opening:
Thanks to BR for providing this!
This gives you R10 insulation over the stairway opening and seals it against infiltration. Since the stairway opening is so large, I would consider using a double thickness of the 2 inch rigid foam to give you R20. It would also be worthwhile to look all around the opening for any other cracks that air can get through and seal these up with caulking or with the polyurethane foam in a can (e.g. Great Stuff).
Pull-down attic stair openings are notorious for leaking lots of air. The stack effect in the house actually drives air through any open cracks. In addition to sealing the attic stair or hatch to the attic, its also important to seal any other openings, such as wiring and plumbing penetrations. See the sealing links for more.