[COcyclist]

1) Any vehicle likely needs a frame to support the belly pan from above. Has anyone built one that attached directly to the car's underbody?
-----I attached mine directly to plastic plugs under the car from the manufacturing process and a plastic fairing in front of the fuel tank and into the bumper plastic at the rear. I added aluminum flashing fender liner extensions in the rear wheelwells to seal off the belly pan and give me other attachment points. I used an external support strap of aluminum about half way back because the factory plugs were too far apart. I drilled NO new holes in the car sheetmetal.

2) How to attach framing and sheet material? Rivets? (aluminum or something else??) Sheet metal screws? Rubber well nuts, aka expansion plugs?? Something else?
-----I used screws designed for metal roofing. They have a hex head and a washer with a rubber washer underneath, and they are self tapping into the plastic plugs. I did use an anchor screw into one of the forward body plugs. I fashioned a couple of brackets out of aluminum strap to support the coroplast in a few places.

3) What framing and sheet materials did you use? What worked and what didn't work?
-----I scored a donated 4x8 coroplast election sign. It seems plenty sturdy, light and best of all, free! It was big enough to cover from the back bumper up to the catalytic converter.

4) How did you deal with the engine bay, between radiator bottom support bracket and the firewall?? Were you able to put a panel over the area, or at least over part of it?
-----My car came with a factory belly pan under the engine, which I replaced with a sturdier aluminum skid plate. I added coroplast wings? to fill between the bumper and the front tires, attached to the front fender liner lip.

5) How did you deal with the exhaust area? I'm considering leaving mine uncovered (it goes down the center till it gets near the gas tank) but what did you do? If car has a rear muffler, did you cover it over?
-----I had previously removed the muffler and put on a straight pipe (It really is no louder than stock in the Turbo Diesel) to give a more room for a flatter upslope from beneath the rear axle to the back bumper. I covered everything except about a foot where the catalytic converter is. Caution! Those things make some serious heat! some places farther back I'm sure the coroplast is in direct contact with the exhaust pipe and I have had NO melting or even discoloration of any kind. Your results may vary.

I did not do A-B-A testing after 2 days of rolling around on a mechanic's creeper to get it installed but I had my first 900 mile tank since I have owned the car. My fuel log shows a very low tank mpg for one particular trip but that was in the name of science. I did some high speed testing of the belly pan and everything held up fine. (Still going strong even in snow and ice)

I would rate this mod as very successful. Mine was minimal cost, totally stealth and has made a big difference in my highway mileage.