Parts I purchased:
1) 1 4" by 8" sheet of 200 grit sandpaper (hobby, craft or hardware store).
2) 1/2" wide double sided tape (hobby, craft or hardware store).
Print and assemble all pieces as shown in "Assembly.skp". Carefully remove the hold down pads on "Base.stl" and "Axle.stl".
Place a strip of sticky tape on the rolling surface of each of the two ramps. Next, carefully stick the sheet of sandpaper on the tape, then carefully aligning the edge with the edge of the ramp. Once positioned and secured in place, carefully trim the excess away using a razor blade or hobby knife. Repeat for the remaining ramp. I've included a photo of a completed ramp.
Comment if you have questions.