Wiggy's has a quilted, lined, machine washable, synthetic insulation product for $9.50 per yard. That's about one half to one third the price of WarmWindow. It does not have a vapor barrier.
"...insulated window covering has as its core Lamilite. The two exterior fabrics are cotton poly blend material."
This stuff might be useful. I may get a couple of yards and try some ideas... Tee-Dee's insulated over-cover, or maybe a custom-shaped top quilt, or a two-thirds UQ insert for my MacKAQ.