: 10. When it is almost fully cured let the air out of the tube.
: 11. Then pull the tube out of the hole.

It will be hard to pull a rubber tube out. Epoxy is a good adhesive even partially cured.

I used to use a short length of Teflon rod for this sort of thing. After cure just pulled it out with pliers.

Not really clear to me why not better to just put a Teflon rod across ( or similar "slippery" plastic) and then do the end pour making sure that leaks around the rob are sealed with something. I may not have followed the reasoning etc of the "c" shape