Cut a round piece to fit the hole close to size. Take a piece of hardwood. Insert it through the hole. screw on the right and left. then take your cut out and lay it over the hole. then use some sealant to seal the gaps. With wood and sheet metal.. that is a common aircraft repair....
You can attach with screws as stated above. even put a screw in the piece as a handle. to hold onto.
You can do all the above without screws just epoxy.
Wrap a string or wire around the patching material. and have a topside attachment point, wood, plastic, metal plate, rod. etc.
If your fear glueing to top attachment piece simply use plastic between, siran wrap or plastic baggie anything.. as a nonstick barrier.
After you get that in place you can pour foam in if you need or want it before you close it up completely.
Several different ways you can patch with only topside full access.(limited below access)
You may drop a few pieces in said hole if this is a new technique,.
It has to be narrow enough to get in but a gap can be left on 1 side to pour past.
Just me, I would epoxy everything to help keep water at bay in future. I'm talking about the insert piece, epoxying/coating prior. let it cure so its not so messy and slick, or do it all at once. like I said, many ways to do this.
Best O luck.
