1) Sweat it out for a few days.
2) Build a platform to make it easier to install your window AC unit on your older windows that it doesn’t fit so well in.
3) Paint said platform so it won’t warp.
4) Install the platform and the Air Conditioner. Also build a panel to block up the excess window area. Make sure everything is nice and sealed so air and bugs won’t get in.
5) Once that’s done, the temperature outside will drop 15 degrees so that you won’t have to even turn on the AC you just went through a lot of grief to install.