Sunday night we knocked together the frame. We built it like a stud wall, 16"o.c. and all. The reasons were so it would be strong enough to support the door on its frame and also so that it would be strong enough to walk across when it's closed.
First thing to remember is that the door itself is not a functional part of the hatch. It's purely aesthetic. We centered the door on the hatch frame and screwed it in with plenty of 3" decking screws. Next, we picked through our stack of scrap birch plywood (leftover from the ceiling project) and found ones that would fit.
Rather than measure and mark, as we did with the ceiling, this time we just held the pieces against the frame and traced. All but two of the cuts were done freestyle with the circular saw, no guides. Time is of the essence, and the actual edges would be hidden against the attic floor anyway. Then Erika marked where the screws would go and I used the ol' forstner bit to drill the 1/2" flat-bottom holes, halfway through. In them, we sank the screws and kept our traditional 1/8" gap for design. The whole idea is that this hatch paneling would match the ceilings.
The thing weighs a ton. It is very, very heavy. Not a very practical hatch. But, we're scheming on a pulley and counterweight system that will make it easy to open.
We are very happy with how it turned out. It looks ultra funky. A door on the ceiling, over a stairway that looks like it leads to nowhere.