I had an old Volvo that had a similar problem... only worse... almost like the Flintstones car. I got a piece of sheet steel, cut to fit and riveted it in place... better than sheet metal screws, IMHO. I gave it a nice thick coat of aerosol undercoating. It was still holding strong when I sold it years later. If I had it to do over, I would probably use heavy gauge sheet aluminum because it would be much easier to work with.