Thank you!

My antenna is already build and strung up to stretch before I lock
everything in place. The holes on the pipes are slightly too large to
provide reliable friction.

I also got email advice (thank you Kees!) to use tie wraps around the
wire but _inside_ the pipes. I'll use a couple of rounds of copper
telephone twisted pair for that. Twisted pair is an interesting
material.

I am testing polyurethane foam with the electrical pipes I used, which
are made of some unknown polymer.

If the foam bonds OK, I will inject it, then stick a ball of crumpled
alufoil into each end of a pipe, and perhaps dab on another bit of UV
resistant glue to block the foil balls in place.