Used a cutoff wheel in the Dremel tool, cut a first hack hole. Cleaned up the hole using files for the straight sides, a sanding drum in the Dremel for the corners, then polished off the corners with a small ScotchBrite wheel mounted in my hand drill.
Spent some time cleaning up the mounting rails for the lens. They (Duckworks) say that you can use the pulled rivets if you don't have a rivet squeezer, but I may borrow my neighbor's. Marked centerlines and mounting rail distances on the skin.
Could rivet the skin now if I so desire. Cutting the skin was much quicker, easier, and less traumatic than I thought it would be.