If you're talking about the MaxAir Vent Covers, they are real easy to install, no drilling into the roof, they attach to the screws of the vent. Very easy to do and you can do it yourself. Here's a link to installatin instructions
for the various MaxAir vents.
There are two types of MaxAir
Vent Covers that you can use.
There is the Standard
Which you can pick up at any RV Dealer and some Wal-Marts, costs under $30US. People have used these with their Fantastic Fans with no problem. Some have replaces the screening with different screening to provide greater air flow.
Then MaxAir has one designed for power vents, like the Fantastic-Fan, itís the Fan-Mate
Costs ~ $80 at Camping World, but is designed to provide greater air flow. But seems like some people don't like this cover because it creates a lot of dirt on the roof because the fins are pointing down to the roof.
Hereís a thread on the Open Roads Forum
that talks about this specific subject.
Personally if I had a fantastic-fan I would use the standard MaxAir vent cover and replacing its screen to create greater air flow.
For getting on the roof, just make sure you have some plywood (or boards) that spans multiple roof trusses for support. I also put some carpet on the underside of the plywood for added protection of the rubber roof.