TImes have most certainly changed...no matter how you fix it. unless its certified? (truth be told I dont even know what that means.) its lawyer time. That is.. if some one gets hurt or worse yet dies. and they determine your shop is involved. same with seatbelts. etc.. Back in the 70's 80's I have done countless seatbelt mods.. now? Not even a chance. to me its not worth the risk. just my 2 cents.

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.