I'm just posting because I haven't seen any one do any kind of full body length skid plate on here with material that will actually last and take some abuse. I'm going to start messing around with the shop car on my free time to see what I can do. My main question some one might be able to answer is there any proven info about mpg and underbody skid plates? Either way I'm going to be adding an underbody plate and most likely cutting out the rear bumper mid section and add a rear valance like I have on my drag car. I know I picked up about 5mph in the quarter mile from cutting holes in my bumper so I can't imagine what kind of difference there would be if I had an underbody plate and cut out the bottom of the bumper.
It's just an idea right now but I plan on making it with a hinge and a few pins holding it in place so it will be able to be removed very quickly like my race cars hood. I just can't find any hard numbers on how much it will actually help. But I guess I'll find out. A few sheets of metal and a few hours shouldn't be too bad.
I have an old Beater '95 Civic with B16 swap, my avg. gas mileage has actually gone up a few mpg from running no rear bumper. This is with out any underbody aero besides that little chunk under the bumper.
Try using coroplast. It is lightweight and relatively inexpensive as well as decently head resistant. The area underneath vehicles is usually aerodynamically dirty. A full under-body skid plate may not be needed as post engine bay most of the underbody is flat. Maybe a partial will do. If you're going to cut out the bumper as well make sure nothing is being exposed behind it that may increase turbulence. The bumper already serves the purpose of directing flow around the vehicle and cutting a hold may cause turbulence which would result in FE robbing drag.
I drove a class 8 straight truck, 40' long and aerodynamic like a brick. Almost all big trucks have a visor at the top of the windshield. Taking that off increased my mpg about 7%. At 90k-100k miles a year that's a very significant improvement. It will be interesting to see what a full underbody does for the car.