Welcome to iRV2.
This is your first post I see.
How old is the RV ?
The solution could be to replace the frame and the vent; instead of just trying to find a vent to fit the frame .
Could you post the measurements and a picture?
EDIT: My vent has the retainers at the bottom too , so I'm interest in this issue.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.