How to tie fishing line to a pole

something more durable and permanent but still adjustable

use 1/8" stainless steel cable and appropriate clips the clips can be had fairly inexpensively
https://www.e-rigging.com/wire-rope-clips.

I walked past something like that today and remembered this thread, so took a closer look. They had simply drilled through some metal posts, run the cable through, and stopped the end with a slip knot, which was secured with one of those clips so it would not accidentally come undone.

This has been fantastic information - I really appreciate the input. We’ve gone with 1/16th braided wire (since it would fit the holes we’ve already drilled in the pipes). A slipknot or clamp may be what we use to tie the ends off - we’ve got some ferrules but not the proper crimping tool.

In case anyone is interested, among other things Ashley has:

There is also relevant material in the chapter on post hitches.