Seriously dude, just go to home depot or some hard ware store, I would say 2" pipe would be good. Buy a 2foot chunk of black abs pipe. And tell them to cut a strip down the middle.
Then just debur the edges with some sandpaper and you have your down tube protector. You can try cutting the thing in half your self. But, good luck, it has a tendency to squeeze your saw as you get further through. Its kind of a pain. tell the home depot people to do it, and its their problem hah.
I dont use one, but I've made them for my friends and they like them.