While building our Q pipe from XS plans, I initially tried to use sheet metal for the ground transition. I couldn't find flat sheet, so I was going to try cutting up some duct. It wasn't going well, then I stumbled upon some left over 1/8 inch ABS sheet plastic I had.
I gave it a shot and it works perfectly! It's a very smooth transition. It flexes just enough with you as you transition on the ramp. Being ABS, it will wear very well. It's very durable. And it will never rust!
You can order a 24"x48" sheet on-line for $24 (plus shipping). Rip it to make two+ pieces to span an 8 ft ramp. The seam in the middle doesn't seem to get in the way at all. My transitions are 5" wide. Check the attached pics and our project page "Zach's Ramp."
You can order it here, http://www.rplastics.com/abssheet.html (located in San Diego for drive up) or check locally for a plastic supply company.
It's the way to go!
Good Luck!