Can you solve your problem by seasonally sealing/insulating the existing unit?

"Is there much savings over ac usingthe fan?"

Offhand, I'd say yes, if the fan produces an acceptable comfort level.

Hadn'tthought about that but possible.

There are covers designed to sit on the attic fan when not in use that are built out of foam that can be laid over the fan when not in use. I built a fan cover for a neighbor and it wasn't bad. I put a 6" wide strip of plywood around the unit and sealed any gaps to start. I then built a cover out of plywood and 2" foam and added some guides so it would slide into place. I ran a small cable from the box to a pulley on the roof over to the attic hatch so it can be raised and lowered.

I would after learning about the prebuilt covers adapt one of these with a thin plywood stiffener for the cable. go that route. On cooler days or evenings the attic fan really feels nice.