Adding insulation WITHOUT tearing up the rig??

A portable AC that vents out a window will draw HOT air in from outside. I had a portable when our central quit in the stick and brick, and while it did blow out cool air it did not work very well at all. A floor fan to circulate the air helped but I was real happy to get a new central unit.
I don't think anything you do is going to make a big difference in very hot weather. Remember you are in a big aluminum box.