I use a small loop of light rope and a prussik hitch.
Fits any small diameter mast.
You can put a small “lanyard clip” or fishing snap swivel on it to attach the wire if desired.
This braided mason’s twine holds well, but if you really want it to have a good grip, use some of the old waxed cable lacing twine, used for tidying up the wiring in valved equipment.
I use a tent line adjuster for strain relief at the apex of the V on my 58 footer.
This is kept in place at the top of the first section with sufficient turns of insulating tape below the adjuster to prevent it moving downwards and sufficient turns of tape above the adjuster to provide friction and stop it flying off the top.
This still allows the top section to telescope into section 2.
This has worked flawlessly even with the top section horizontal in the wind.
I use a sotabeams tactical mini and use a rubber tap washer , available from hardware stores or plumbers merchants which fits a little way down the top section and stays in place.
Andy
MM7MOX
I either use the antenna insulators sotabeams sell which has been suggested by others but may not work for you? Another solution I’ve used in the past is aquarium tube. Just slice a bit and slide it over the top of the pole and the wire creating a snug fit.
Good luck!
For ham radio antenna work, this is one of many useful knots.
To learn about knots, I find climbing folk are the best people to listen to; they typically bet their lives on what they are telling you 
Try the link below. Start at 16:48 for the Prusik …
73 Dave
There are several variants and alternatives, as climbers often complain that the Prusik is difficult to untie, especially when it has had to hold a lot of weight (which it often does, given how it is used in climbing).
I haven’t found that to be a problem with antennas. But I’d suggest that you tie it the reverse of the photo I provided. That is, put the knot that closes the small loop of line in that vertical portion of the Prusik just to the right of the mast. (Hold the knot in one hand while looping the remaining string through with the other hand.) That gives you something to grab onto to pull the Prusik apart if it gets too tight.
One reason I prefer this approach is that, if the tip of your mast snaps off (yes, it has happened), the Prusik can still be used on the remainder of the section, or moved to different pole, because it doesn’t need to be carefully sized to the mast to hold the antenna up.
