Adding a new roof vent (& cutting the hole for it)

I can tell you this . DO NOT CUT A HARD 90* IN THE CORNERS!! Make sure you make about a 1" radius into the corners. It will prevent cracking. Most critical with a hard surface like fiberglass. Regardless please take my advice any roofing materials that take that kind of abuse will crack over time in the corners unless the are rounded.
Good luck