With orders of $25 or more, Amazon offers the Duck Brand Window Air Conditioner Insulating Strip for $2.88 with free shipping. (Walmart has it for the same with free in-store pickup.) That's the lowest price we could find by about $2. It measures 42" x 2.25" x 2.25".

Note: This item is part of Amazon's "add-on" program and can only be purchased with orders of $25 or more.