Hey Rockman,
Knowing that you've got good fab people, I'd be looking to have them make a bracket to hold one of your car's airfilters in the box, as the dust filter for the fridge vent. That way you've always got a spare filter for the car if you need one.
Personally, I'd got with one of those little
solar powered marine fans on top, and just a normal vent down lower, so you can take advantage of the natural convection effect to draw in cooler air at the bottom. You'd want filters on the inside of both vents, but that shouldn't be difficult. You *may* want some way to be able to seal a cover over the lower vent, for water crossings, etc. IE, like Teabag fabbed up for his Jayco door vent. Again, I'd expect your guys to be able to knock something like that out in their sleep.
I like Sablesoft's setup - I think you could get somethign pretty similar, and it would suit your requirements.
Good luck!
Matto