There are generally a few of those access panels listed on E-bay. If you don't find one check back in a week or so.
I personally would not cut the whole opening at once. I would cut a 4" square hole , then reach in there and see what you find. If that looks good I would cut out a few more inches, then look again. There are many surprises lurking behind that skin. If you start small you may find a problem but, still have time to relocate your opening to make it work.
Oh yea, Welcome to the insanity!