i would say start with a dremel or jig for the fiberglass/abs....then use a drywall saw for the foam.....a tight circle is going to tricky with a sawzal imo.
can you get a hole saw that big?
if you're stuck with the sawzal then cut/grind the blade so it's thin (the back side....not the thin side), it'll make getting an accurate curve easier (thick blades don't turn tightly easily) i think that made sense
