While it is possible to use frame tubing (of course as a framebuilding teacher I have hundreds of short pieces of scraps), they are a bit of a sloppy fit. The very best option is to use steel tubing that has a wall thickness of 0.058". The outside diameter (ø) will be 1/8" larger than the tube you are placing this tool on top of. For example if you are using the tool on a 1 1/8" down tube, the tool itself would have an OD of 1 3/8" with a wall thickness of 0.058". This is commonly available tubing from steel tubing suppliers. For example Wicks Aircraft or Airport Spruce are 2 examples of places that sell 4130 tubing in those wall thickness. They sell it by the foot (or more).

The purpose of 0.058" wall tubing is because they are a slip fit that can create telescoping tubing with just the right amount of clearance so the smaller tube can fit and move inside a bigger one. I don't know anything about and have never tried to use plastic tubing. It will work if a split piece fits snuggly on top of a tube. Of course I have all the right tools fo make tool #2. I have wood blocks to hold the tubing while it is cut. I have a lathe and/or a belt sander to square the ends of the tubing stub before I split it lengthwise in half.