Instructables
Picture of Whole House Fan Cover
One of my fall chores is to insulate the whole house fan in our house.  Previously I built a box in the attic out of foam boards, but using it is a pain because I have to empty out a closet to get to the attic door and then  crawl through deep blown-in insulation to get to the box. 

In our house, the fan comes in through a hall ceiling. It would save a lot of trouble if I could put an insulation cover on the fan from the hallway and avoid the trip to the attic.  A quick search on the internet shows some nice looking products, such as http://www.batticdoor.com/WholeHouseFanCover.htm.  The ones I found cost at least $40 after S+H and consist of a thin flexible insulation blanket.  I wanted to spend less money and have more insulation.
 
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Step 1: Get the materials

Picture of Get the materials
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Measure your fan opening.  Mine was 36" by 31".
Buy a sheet of 1 inch foam insulation.  I had to buy a complete 4x8 foot sheet for a little less than $20, but maybe in a larger town with better DIY stores you could find a 4x4 sheet.  Hopefully I can find someone who can use the left over foam.

Cut the insulation to size.  I cut mine to 36" by 32" to take advantage of the factory scribe at 32".  Fairly clean cuts can be made by scribing with a utility knife and snap using a straight-edge.  The picture shows my first cut which is not so nice.  It cleaned up nicely with a Dremel tool however.  This edge faces the wall so it is not very visible anyway.

Finally, buy a box of sticky back Velcro.    Amazon lists a box of 15 feet of 3/4 inch Velcro for about $15. I had a box with 20', but you only need about 12 feet.  Generic versions of Velcro might be cheaper; I didn't check since I had some on hand.
Very nice solution! Much better than crawling around in the attic!