How do I draft proof a “drop down” loft hatch
I've seen solutions that go over top of the drop down-stairs.  You can find them by searching for 'attic stair insulation kit'.

If you were to go completely home-made, you can get sheets of rigid styrofoam, then make a box that you can drop over the hole before you put the stairs back up.  If the floor is flat and smooth, that might be enough on its own, otherwise you can put down some weather stripping and find a way to secure it down.  (maybe eye hooks?)