put batten round the hole for the hatch to seat against, with draught excluder on it. That will seal the draughts and moist air.
I haven't seen a picture of this ladder you mention, but I'll guess the hatch swings down. Put insulating foam slab on the underside, and decorate it or cover with hardboard and decorate if it looks better. If the hatch door is hollow you could put insulation in the space, or if you feel like it make a new hatch door with a sandwich of ply and insulating foam slab.
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