I bought a used door (in good shape) to install on my trailer.
(Like many other stuff, I had no time to install it yet).
It is made as you describe, thin aluminium sheet backed with styrofoam filler, except mine has rounded corners.
I drilled a few pop rivets and it all came apart, I took the aluminium sheet and foam off, and I plan the use the cutout as the door.
The door is about 36" wide X 12" tall or so if I remember right, perfect size for me. It had two tiny and flimsy latches at the bottom, I bought a T handle lock to replace them and will fabricate a locking system with bars, garage door style.
To cut the opening I guess my oscillating multi-tool should work fine, however a jigsaw should work too and will be necessary for the rounded corners.
Couple years ago we cut a square hole on the side of my father's motorhome (a vintage Dodge Travco, a molded fiberglass RV too) and we used a 4" grinder with a cutting disk and it was easy (and the FG shell was thicker than on our trailers, maybe 3/8").
They say "measure twice, cut once", but in this case I would say "measure about ten times, think about it again, measure once more, take a deep breath, then cut" !!