I already mounted one Maxxair II vent cover to the rear vent with the little fan in it. The mounting system worked well there. It is easy to reach inside the vertical part of the opening flange and tighten the nuts on the mounting bolts.
The double walled Fan-Tastic Fan mounting would require disassembly of the fan to mount the nuts on the bolts, and I didn't see how to do that. The alternative would be just some sheet metal screws screwed into the plastic flange. I didn't like that idea.
So I decided to go with the Fan-Tastic Ultra Breeze vent cover. The mounting brackets slip up under the opening flange. It requires removing and reinstalling 4 screws in the roof and re-applying some sealant, but I don't think that will be too difficult.
This shows how to mount the vent cover.
https://youtu.be/aAs2GsUTdK4The standard Fan-Tastic vent cover does not have the mesh screen like the Maxxair does. The mesh screen is an option. That may be part of why the Fan-Tastic vent cover is less restrictive of flow than other vent covers on the higher volume fan.
This shows a significant air flow restriction for other vent covers compared to the Fan-Tastic vent cover.
https://youtu.be/p1fhkbl_1yQI started calling the RV dealers starting with the closest. The fifth call found a vent in stock. After a 100 mile round trip, I have one in hand. Looks like I'll be waiting a couple of days on the weather to install.
2013 F150 XLT SCrew 5.5' 3.5 EB, 3.55, 2WD, 1607# Payload, EAZ Lift WDH
Toy Hauler: 2010 Fun Finder XT-245, 5025# new, 6640-7180# loaded, 900# TW, Voyager wireless rear view camera
Toys: '66 Super Hawk, XR400R, SV650, XR650R, DL650 V-Strom, 525EXC, 500EXC