Our 2007 Phoenix Cruiser has two power roof vents, a generic version in the bathroom and a Fantastic in the bedroom. Outside on the roof they are very similar in size & shape, and how they open.
I wanted rain-&-wind covers for them but didn't want to spend a fortune. I found the Aero-Flo product sold at Walmart
HERE for an affordable $19.69 plus sales tax. I bought one on-line and had it shipped to my local Walmart to save around $7 in shipping costs. The unit installed extremely easy. Tools needed was a drill with a specific bit size and a screw driver. NO holes go into the roof, only in the side of the raised frame of the vent cover which was important to me.
I installed the Aero-Flo over our bedroom Fantastic fan. During our last RV trip, I was impressed with how good it works so I just bought a second one for the bathroom vent and will have it installed prior to our next big trip.
One thing important to me is for the vent to sit lower than the a/c unit (which it does) because the a/c unit just clears our garage door opening.
For you on a budget, the Aero-Flo is a great solution for a roof vent cover.
I also need to mention that opening the roof vent tight to the Aero-Flo cover, eliminates the noise of the cover clanking in the wind. With the roof vent half open, it might also eliminate the noise but I always opened the vent to contact the under-side of the Aero-Flo.