Maximally Sealing Window Air Conditioner
I also like the rigid foam @jimmy fix-it commented. it is easy to cut to size. I did see a person that used plastic wrap and injected expanding foam, the plastic kept the spray foam from sticking to the window. He folded the plastic in 1/2 and used the nozzle to shoot the foam in but not two much the plastic keeps the spray foam from sticking to the foam board and window it also locks everything together once dried and is easy to use a box knife or steak knife to cut the spray foam on 1 side then it pops out the foam board can be used again next year.