I think I would try installing a seal around the top edge of inside of the freezer compartment. It looks like you have a fairly uniform gap between the side walls and the inside of the lid. You could use a high quality neoprene type weather stripping (not the foam kind) that is like a tube with a hollow space in the middle (it comes in various thicknesses). 3M makes a very strong weather strip adhesive that you could use to glue it to the side walls with.
SV Laurie Anne
1988 Brewer 40 Pilothouse