I have several brand-new-in-box, never-been-used Frost King Attic Fan/Whole House Fan Covers for sale.  These covers will fit attic fans and whole-house fans up to 48"x48" (trim to the size needed) and will seal air leaks around fan louvers in the ceiling to reduce energy costs in the winter.  These covers are easy to install when the cold weather arrives--just cut the cover to the size desired and attach using the included loop and lock (velcro) tape--and easy to remove when spring returns and the fan is used. 

Cost:  $9.00 per cover