Lisa made the sock to have the exact rectangular opening of the AC unit on one end,, and the measured outer window frame on the outer edge.
I used thin aluminum strips and VERY short sheet metal screws to fasten the "sock" to the AC unit. The outer edge has 1/4" bungee cord in a hem and one of those squeeze cinch thingies.
There is two fabric layers with some insulating foam in between them to help (marginally) prevent air-to-air exchange.