you have good access to the back. I would recommend you reinforce the corners of the cut-out a bit, which is very easy:
Buy a couple of feet of 4" wide fiberglass tape and some epoxy/hardener. Draw 4" diameter circles (27- degree circle-segments to be exact) at each corner with the corner as the center (after some cleaning
with aceton or even a little sanding
too). Cut 4 circles out of the glass (mark first, incl. the center of the circle). Mix some epoxy and paint
it on the marked area and immediately put the fiberglass on. a 90 deg segment will overlap the panel... using scissors, just cut into the circle a bit so that the rest goes nice and flat on the epoxy. Use the brush to add some more epoxy on the glass but not so much that it starts running. After some time the epoxy will gell, it will still be sticky. Use the scissors some more so that you can fold the fiberglass away from the panel onto the part that was epoxied (it will stick there on the sticky epoxy). Apply more epoxy over all of it, incl. the doubled up part.
This will disperse the stress riser at the corners and I estimate it to be an 1 hour job.