I did these the exact same way I did the skirts for my car (linked to in the first post)... I truly am sorry for not taking more photos of the steps during the process - I just get going and want to keep working. I need a photographer to follow me
.. Anyway, I use aluminum stock from a local hardware store, home improvement store. I'm not sure about the exact gauge, but it's not anything too heavy. I cut 3 aluminum "L" brackets, about 2cm wide. I then mount them to each side of the wheel well, and the top (I use little self-tapping metal screws, and make the holes in the L-bracket before mounting them. I then mount them to the wheel well, just tapping directly into whatever material is in that area of the wheel well. For this car there was an existing mount hole available in the aft of the wheel well, so I just used that with the stock screw. Reusing this existing screw hole made it impossible to use a regular L-bracket because it is set back into the wheel well too far for the coroplast to stay flush with the fender around the wheel well. For this I made my own L-bracket out of 1 inch wide aluminum stock (very bendable) that reached out far enough to make things flush. I also bent it so that it is at an angle greater than 90 degrees to make the bend in the coroplast less dramatic around the outside of the wheel. Once finished with all of the brackets, I shape that 1" wide aluminum from the front bracket to the back bracket so that it sits as closely and aerodynamically graceful as possible to the wheel. Then take the wheel off, put the band back on, and get the coroplast sheet. Hold the flat portion of the sheet to the bottom of the aluminum band, hold it to the fore side, reach behind and trace with a marker, then carefully keep the coroplast in the same vertical position, roll it over the rest of the band to trace the remainder. If you don't do this and just trace the wheel well, your skirt piece won't be wide enough to accommodate the curvature of the aluminum band, and you will have wasted a reasonably large piece of plastic. Then cut, paint (optional), and screw it in. I'm a lot better with tin snips than a razor, but that's up to you.