Walmart offers the Duck Brand Window Air Conditioner Cover for $3.95. Opt for in-store pickup to dodge the $5.99 shipping charge. (Amazon charges the same with orders of $25 or more.) That's slightly less than our February mention and the lowest price we could find now by $24. It measures 27" x 18" x 25" and includes straps and fasteners.

Update: It's now out of stock at Walmart; however, with orders of $25 or more, Amazon still charges $3.95 $4.99 with free shipping.