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!
That is the correct terminology as far as I'm concerned. Rolled foam sounds great, the more accurate with smallest gaps the better. Anything to stop the airflow will be a good start and then anything insulative will increase efficiency.