Frost/Ice is a very good insulator, which is why igloo's and snow shelters work. Front opening refer's allow all of the cold air to spill out at the base, which is why they aren't so great aboard ship. This also allows warmer relatively humid air into the box where the moisture in all that air ends up on the cold evaporator plates.
You might try adding overlapping vinyl strips just inside the door seals
to minimize cold air loss. While we were looking for the boat we have now we were aboard one yacht that had done that--and partially secured the bottoms of the strips with a length of shock cord, and the owner/seller claimed it worked very well.
As for heat guns
and defrosting, I raised that very question with our AC tech and his response was that the heat raised the temps/pressure in the plates too much and suggested that we use tepid water
only. A couple of pours and the frost sheets
off, although that could be a tad messy with a front opener, eh?