I had long strips of Acrylic cut to the proper height for the windows, with about a 2 - 2 1/2" overhang past the edge of the opening. Then I rough cut the vertical sections (Used a router). then used a belt sander with rough paper to scult the corners to get the shape I wanted.
I used 3M adhesive tape (3/4" I believe) primarily to hold the new oversized acrylic windows in. Then I left a 1/4" - 1/2" gap outside the tape, and then ran a large bead of Dow 795 to further adhere and seal the edge. I ran a finger around the edge to bevel it off to a clean (approx 45 degree) edge.
I am extremely happy with the job. It has been a year now, ad no leaks.
Made the boat look much better and newer than the old, cheap, white plastic frames.
On the strength or the 3M tape. I had lightly set one into place, and hadn't even pressed it hard to the tape. Of course the Dow 795 was still wet (not adhering). I realized it was the wrong tint, so it had to come out. I had to put my shoulder into it wit halot of force, from inside the boat, in order to break it free. I would imagine that once the Dow was dry as well, with well over an inch of pressed sealant, it is in there good.
By using the 3M tape, I did not have to use spacers to allow the correct thickness of D795. If you had not read about this, to get the Dow to be able to flex and expand with the acrylic, during temperature changes, you need it to have some thickness. You do NOT want to press the window tight against the fiberglass, squeezing out virtually all of the Dow sealant.
Tried to upload a couple of pics, but they failed.