Add an access hatch to my attic

Last winter, I cut a ~4”x4” inspection hole in the drywall that makes up the ceiling for the closet in the addition with the inaccessible attic. I wanted to look for wires or anything like that. I turns out that there’s plywood decking on the other side of the roof joists. It’s not nailed down or anything, and when I push it up as far as I can (only a few inches) there appears to be radiant barrier on it as well. So no cabling in the family room for now. D’oh.

My house has a very well done ~400 square foot addition. However, there’s not access to the attic above it. I have no idea how much insulation is in there. Also, I’d like to get in there to do some cabling.