Ok, so what you want here is one of two things:
1- Coffin: Larger version of an ice chest. Cap the opening with a coffin made of 2" extruded polystyrene board. Rig a stick or rope to open from below with ease. Cost: under $50
2- Cork: Build up a cone around the fan opening that is airtight from below. Shave a section of xps board to fill the opening. If you're sly, this one might work by gravity alone. If not, rig as above example. Cost: under $50.
Both of these designs can be sealed magnetically or with fur. I prefer shaggy felt.
Or, you can go buy a self-sealing, microprocessor controlled, automated, high r-value whole house fan blower for thousands of dollars.