Lee B wrote:

I'm looking for a magnetic cover to block the shutters on the whole
house fan (from the downstairs side).

snip

I cover mine with one of those clear vinyl kits they sell for sealing
windows in the winter. Double-sided tape around the edge, attach the
vinyl, a little heat from a hair dryer pulls it tight and smooth

The styrofoam and magnet idea is interesting except my frame is aluminum