I thought it might be smart to buy some foam board and cut it to fit in the doorways. Shortly after purchasing the foam board, I went to a neighborhood class on emergency light and heat. The instructors suggested taping those space blankets in your doorways. Duh! That would have been a lot easier and cheaper than foam board! Oh well, but I do think these will do a better job.
We had enough for solid pieces in some of the doorways.
But others I had to piece together. It was easiest to cut with a utility knife like you're cutting sheetrock.