How about using an anchor insert that expands on the unseen side, that has a threaded machine screw or epoxy (J,B Weld) a threaded insert in place. If needed you could beef up the wall thickness by epoxying a thicker piece of metal to the wall where the fastener will go. Just a thought.
Married To The Navigator
2005 Red GMC Duramax 4X4 HD Crew Cab
4" Magnaflow, Banks Air Intake
Firestone Ride Rite Air Springs