I used this one:
"Duraflo 620808COMB Combination Gable Vent, 8-Inch X 8-Inch" from Amazon.
It actually comes as two pieces, an inside and outside. I used the outside one for the upper side vent and the inside one for the underside. It's flexible so it conforms to the trailer's shape and doesn't need painting
. And it looks pretty nifty, too.
I don't know what you might use for the main front vent except for the one Casita
uses for it's 16'.