Making and using the tube makes a creative activity for the weekend - in an hour's work, you could make enough knitting pipes to keep your kids, and all their friends, constructively occupied for ages.
I used power tools and a blowtorch, but you could also use hand tools and sandpaper.
As a bonus, I'll also explain how to use the pipe. Even better, there is now a 3D printed upgrade to the idea (see step 7)!
Step 1: Materials
You could use any tube you like, as long as you have the tools to deal with it.
Step 2: Drill
I used a 3mm twist bit, which cut through the tube quite easily, but did slip a little on the smooth PVC. If you have a wood bit, with the small spike at the tip, it won't slide around as much.
To drill the pipe, I clamped it gently in my vice. If you are not using a drill-press, do not be tempted to drill right through the pipe, because the tip of the drill bit will wave around once it pierces the pipe wall, and hit the wrong spot on the other side.
Step 3: Snip
I used the same snips to roughly round off the corners of the tabs.