Adding insulation WITHOUT tearing up the rig??

Doxie Box
You mention that it is a used rig. It may seem off topic, but check the roof on that rig. It should be snow white. If not I suggest doing a roof coating. If the rig is over a few years old the roof may need resealing (for water intrusion) anyway. Most coatings are very white.
White as opposed to dark spots , dirty grey etc. will make lot of difference in a hot sun.
Also make clean both the inside and outside coils on the AC.

Old Uncle Dave