Rubber "carpet runner". You get it at home depot in the carpet section. It will be on a rack on a huge roll, and the guy will have to come cut some off for you. It is 24" wide, and $1.80 per foot. It works really well, and it holds up since it is designed to be walked on. I have used it on many panels in the past, and I also just used it to make a bezel for my Amazing Maze. One side of it has vertical lines, and the other side has sort of a diamond pattern to it.
Another good choice there is the black plastic stair covers. I used one of those on the Artic Mini. They are big enough to fit most smaller classic panels, but they are a bit thick, and don't really bend, so they are only suitable for panels without bends.
The really good thing about using either one of those is that you don't need any sort of plexiglass at all.