I've got my front passenger wheel off to get at my heater stuff. I'll snap a photo later today and post it to show you the shape of the inner panel. The pieces are simple, flat sheets of fiberglass, similar to flat roofing pieces from Home Depot/Lowes. Mine are about 1/8" thick and are fiberglass-bonded along the top inside of the wheel well to hold them in - I suspect that you could use Locktite bonding adhesive or even "Liquid Nails". I ran for years without them and then finally decided to install them. It wasn't a big deal. I would just make a big paper template for the shape you want/need, out of Kraft paper or some flooring underlayment paper and use that as the pattern to cut out what'cha need.