Check out horse trailer windows. I have not installed one yet but I've heard they fit. I'm looking at these two
Horizontal vented
https://amazon.com/dp/B01347A67C/?coliid=I2RJQG0FO34JVJ&colid=1TU5BEJ8FL6I3&psc=0&ref_=lv_ov_lig_dp_it
Vertical vented
https://amazon.com/dp/B013479YAC/?coliid=I5J93TJK46ZHC&colid=1TU5BEJ8FL6I3&psc=0&ref_=lv_ov_lig_dp_it