I have the Landy Glass fitted as shown by Turktowner, and they don't replace the whole panel, as they are bonded to the side panel, but it does mean you can cut out a square and wouldn't need to be too precise.Just need to remember what ever you cut out, it will be visible on the inside, and if you have CSW interior, this is profiles to suit the standard window set.
Landy Glass provide a edging beads to fit to the aluminum edge which makes it look nice and neat.