The Warm Window (
http://www.warmcompany.com/index.html ) can be purchased at JoAnn fabric and gift stores and comes in either the cream color or white. I found the snaps at a Tandy leather goods store. I purchased a small semi-industrial sewing machine for around $500 that I keep in the RV for just such occasions. I've also made thermal bags to keep drinks cold, carry groceries home in and keep frozen food frozen when I defrost the freezer.
The snaps are attached to the front trim with 1/8" pop rivets and washers on the back side. I seem to remember having to trim the head of the rivet to a smaller diameter to fit inside the snap. The holes for the snaps were made in the Warm Window using a hot soldering iron tip to melt the hole.
The bedroom windows, bathroom window, shower skylight and FanTastic fan opening all have these custom covers. I believe the Warm Window material is stated as being 80% efficient so two layers should be 80% of 80% or 96% efficient. Perhaps someone in HVAC can tell us what the R value would be? Per the web site, it should be around R13 for two layers. Pretty good, huh.
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Have Fun!! Mark & Donalda 04 Horizon 40WD no TOW 90,900+ miles and counting
Triumph Bonneville & Susuki S40 on the back