Found the material for the full belly pan on Craigslist, cut a deal with them on the 4 x 4 pieces and I'm going to pick it up tomorrow!
Sheet Aluminum
Doubt that I'll actually get a chance to work on the car at all until the following weekend, but at least I'll have the materials on hand now. Unfortunately this one won't be a freebie like most of my mods, but it'll be an easy installation.
I plan to just screw the 4 x 4 pieces straight to the aero panels on both sides, and overlap the pieces, starting at the rear and working forward to the front, like fish scales.