LlBr

Hint: Go to the back of a carpeting store.

Look in their dumpster. Your eyes will tear up with joy. Get rubber-backed carpet scraps or the foam padding like I said.

Ucube

Nice DIY! Looking at the pics, though, I'd be concerned about accidentally going over the tire stop as it's not very thick. Without a spotter, I might lose track of which section of the ramp the rear tires are at as I back up.

vinnysca

You don't need a spotter. I backed up onto it with my door open and I visualized the rear tires by leaning out the car. You need to back up slowly. Plus you need to be quick footed in case you need to brake the car.