Cover the inside of the window with paper except around the window edge use
release tape.  Apply a two bid fiberglass tape around the window for your
window ring.  Let it cure. Trim to final shape, then wrap it with 1/8 inch
backing foam and cover with desired  upholstery. Remove release tape and
attach with velcro. You can put in dummy velcro pockets on the window before
applying the two bid tape and thus have a tight flush joint in the final
product.

Bob Smiley
N94RJ