My apartment has Frigidaire built-in air conditioner holders. I did not buy a Frigidaire AC unit because the size needed to perfectly fit the holder was unnecessarily large (and expensive). I have a GE unit that's smaller and does a perfect job cooling, but it lets in lots of air from outside. I've use rolled foam but know there must be a better way. What's the best way to insulate from the outside (and if that's not the right terminology, please correct me). Searching for help!