You have two sources of heat loss.
- Through the door panel itself
- through the gaps around the door.
You need to address both of these.
The air gaps are often bigger cold culprits, and do not show on a thermal camera because the allow cold air directly, not having cooled surfaces to show on the IR.
Ceiling access door, that opens upwards?
Tack a carpet or thinner insulation blanket or even styrofoam over it, with about 6 inches overhang on all sides.
This gives moderate insulation and very good air seal.
Larger or thicker insulation would tend to jam the works, unfortunately.
Vertical door, or ceiling door that opens downwards?
Nail a larger piece of board/plywood/whatever over it, that again overlaps the gaps.
This provides both an extra layer of insulation (even if just a 1/4 inch of wood) as well as sealing the air gaps on the sides.
For both of these, a generous layer of insulation can be added to the attic side of the door, as long as it does not interfere with the opening, to reduce heat loss through the door itself.