Trace a line where you want to cut, and with a 5 inch angle grinder and aluminum zip disk start cutting. The 5 inch angle grinder fits in one hand so you can start cutting using 2 hands, once the cut is started the disk will follow your line and you can use 1 hand and steady yourself using the other hand. The round corners would be trickier. Cut all your straight lines first and then using a jig saw with a medium wood blade you can do the curves. Once your opening is cut out, you would have to make new curved ribs for the cutout. You could probably make the ribs out of wood since your a carpenter. Once the new opening is finished i would make an aluminum trim that attaches to the new window and overlaps the outer shell. Then use some type of closed cell foam tape or seal and glue that to the inside of the window trim. When the window is closed the foam tape would be compressed against the outer shell and create a water tight seal. You would have to have some inside hinges almost like a car hood hinge. You can have aluminum trim rings made to hide the cutout edge. Theres a company here that makes rv windows and they could have trim rings made for you.
Remember you have to cut your inside hole too.
and have trim rings on the inside.