So after cleaning up the fiberglass with water and some clean rags, I used acetone and rags to clean it up more.
After it was all cleaned up, I broke out a sharpie marker and marked the fiberglass where I wanted to trim it down.
A little work with a grinder later and I had a fairly neatly trimmed back to the sub box.
Quick wash down with some water later and it looked pretty nice. Just need to let it dry and then wipe it down with a bunch of acetone before moving on to the next steps.
You can see how thick the fiberglass is here. It’s heavier in the corner than elsewhere, but it’s at a minimum 3/16″ everywhere. That should be plenty.
So I attached fleece to an MDF ring and glued the fleece to the back of the fiberglass. I’ll soak the fleece in resin after this point to finish up.