One way vent for fridge

I have a Trak Shak. It has a Chescold (Finch type) fridge in the
fridge caddy which I have found relatively useless on 12volt, but
excelent on gas and 240v. I have just fitted a 12 volt computer fan
which is doing a sensational job of pushing the heat off the
condenser coils and I think may be workable whilst travelling,
albeit with disconnection at the Andersons whilst stopped for any
length of time. Batteries are not a huge issue as I have duals in my
100 series plus one in the trak shak. My problem is that the Trak
Shak is very well sealed against dust and moisture and whilst I dont
want to lose this the heat dispersion by the fan is generating
additional heat throughtout. For instance, the lid of the fridge is
getting hot (outside only thank goodness). I have seen one way vents
before, typically for gas in caravans I think but am not sure how
workable this is in the Trak Shak. Is there such a thing that will
vent the internal air without letting in dust and water?

Please note I have also posted this query already at yahoo campertrailers group and have read all of the previous posts I could find here about 3 ways.