elegant attic hatch design for several purposes
The standard solution for an insulated hatch cover has it resting on trim flush with the ceiling.  You should pound or pack out the framing for the hatch so that it is tightest at the bottom (where it meets your ceiling).  Add weatherstripping to the top of the trim on which the cover sits to prevent airflow when it is in place, and the insulation on top of the cover will prevent radiant/conductive heat loss.