Some photos of cutting window holes. You can see the three inches of foam make for a daunting trim ring. As far as I can find, rv trim rings this deep don't exist.
Some thoughts on things:
The notch to access the fasteners for the window, just enough to get your finger in there to hold the nut. When its "done", as in I don't want to take the window out and its sealed, I'll stick a couple strips of foam in that crevice, or maybe some expanding foam.
The interior metal facing the foam panels is bone dry. This reaffirms my thoughts that bonding the foam to the steel is unnecessary, and in some cases bonding might trap moisture.
There is enough elasticity in the foam panels to move the cutter head of the sheet metal air nibbler.
Cutting the foam out was actually harder and more time consuming than cutting the metal out.