Go to
https://www.etrailer.com/dept-pg-Enclosed_Trailer_Parts-sf-Vents.aspx to find Pop-up, or "Salem" vents in steel or aluminum, for $35-45.
I had considered getting those as my side vents, but opted for another type, that I had seen on semi-trailers, when I ran a receiving department. They never showed signs of leaking, so I got them instead.

- my side vents from e-trailer.JPG (74.94 KiB) Viewed 4569 times
I put screen material inside the butterfly opening, and modified one, used to cover my A/C exhaust ducting. They work great, no leaks.
Also, I put reversible computer case fans on hinges to be able to swing in front of each of the two main cabin vents, in order to pull air in, or to direct air out (keeps dust out!). They can run forever on 12vdc battery power, since they draw miniscule amperage.


