Also looking at venting my fridge box after our shake down trip the other day found the poor old fridge was struggling to keep anything cool. Have ordered a large computer fan and will be lining the box with air cell insulation.
Lowflyer,
You'll want to vent on the side up against the trailer body.
And you'll need a flat air filter on the inside and mounted low.
by flat I mean a filter from a commode or similar that fits onto a flat surface.
Some folded flat bits to hold it, and computer fan would go on the outlet which is high on the side of the box on the same panel.
You can get louvre vented panels from a caravan spares place, or just a pattern of holes in the box wall.
Then your gunna hafta remember it, when you take the trailer swimming.