I’ve heard of fiberglassing over the holes on the backside of the board that way it doesn’t create hump where the material goes.

But there would still be a hole or indent on the surface.

sorry. I should have said more detail. The one way I was told was like above. On the side your fabric would stick to, but tape over the holes. Put the tape one direction then another layer 90 degrees to the first. Use fiberglass and resin. The resin will soak through the glass and fill in the void. And can be sanded to match later. If there’s a low still use filler or more resin. I haven’t done this myself but had a guy tell me when filling the dome light hole this is what he did.

I would use clear 2 inch packing tape to bridge the hole. make sure it's level, and apply headliner material. The hole is still there but it's covered. This works well on smaller holes, thats a pretty big hole.

I would line the good side with some plastic. ie garbage bag or drop cloth plastic. then from the back side use a can of spray foam. fill in ... and not too much. wait to dry. cut off excess.. remove plastci from front side. wire wheel to blend in. Then replace headliner. easy peasy.

I did the fiberglass bit but I think Sam's idea would have been quicker and easier. Thanks everyone.