Your goal is two-fold: to stop air flow and to stop heat loss. To do
it "right", you will need to build a frame of 1"x6" materials to
insert in the existing hole to retain your existing ceiling
insulation. Once this is in place, you will need to frame a "lip"
with some 1"x3" material. This will give your hatch something to sit
on, and you can attach some foam gasket on this lip to provide a seal
I would recommend you build a hatch cover out of wood the size of your
new frame, minus 1/8" on each side for easy opening/closing. You can
use construction adhesive to attach blue or pink styrofoam (the dense
stuff) to the hatch cover. Alternatively, you can build another frame
and attach it around the edges of the hatch cover, then fill it with
fiberglass or rockwool insulation. A couple of screw hooks will allow
you to "batten down the hatches "so-to-speak."
The following links may help you:
Mr Fixit eh