A fine blade jigsaw is my preferred tool. When you removed the window from the old door you can easily see where the cutout should be. To keep the adhesive fresh, store it in the freezer.
I agree. Just got done cutting in two RV windows, one in the sliding door and one across from it.
Then I cut thru the metal partition(in my van) separating the cab from the rear making a passageway.
Also cut refrigerator vents and a stove pipe hole.I got better at it. You can use an angle grinder with a very thin wheel amd the metal will cut like butter so be aware, things happen fast and you could easily remove our thumb. I wear leather gloves, plastic eye protection and a mask. And ear protection!!!
Now you will be able to be precise with the tools.
I used the angle grinder for the straight cuts and the jug saw for the rounds but the jig saw works well too, it is slower is all and maybe a broken blade or two.
Measure twice, cut once.
Make a line, follow the line.
I made patterns too, sometimes that helps locate yourself.
It's nerve racking work. Having a second person would be nice but I wasn't that lucky.
bill in tomahawk