That looks like an interesting grate, Dave! Just be sure you put a screen in it to keep the tiny flying critters & creepy-crawlers out ...

...! What ever you use, make sure it has enough square inches to allow adequate air flow ... the owners manual for your 'fridge will tell you how much.
As far as side vents go, I'm kind of partial to some of the engine cover vents on inboard boats ... check out almost any marine supply store ... but I've also used foundation vents with a nice drip cap over:

This one was a 2-way (12v/110v), so didn't need an exhaust port.
CHEERS!
Celebrating
Retirement after over
32 Years of Building, Promoting, Supporting, Supplying, Living the Lifestyle, and Loving
Teardrop Trailers!"Life Moves a Little Slower When You're On Teardrop Time"The nature of Life, itself, is change ...
"Those who matter, don't mind, and those who mind, don't matter." 