DON'T EVEN THINK ABOUT USING A CLAMP!
THIS WILL DAMAGE THE POLE!
USE A SMALL, STAINLESS STEEL PULLEY
First tie a rope to the pulley, Then fasten it to the pole using the rope. The Spiderbeam 2mm Kevlar rope is perfect for this task.
If mounting to a thick part of the pole, you can use a strap.
The thicker lower segment prevents the strap from sliding down.
(Note: normally there would be a clamp where the strap is shown and the strap goes above the clamp.)
If mounting higher up the pole, simply tie the rope to the pole, just above the clamp.
The small pulley weighs very little, the rope even less.
Weight is a fiberglass pole's worst enemy, especially when mounted near the top of the pole.
Using a larger pulley is very dangerous.
Using a metal clamp to fasten the pulley is even more dangerous because it not only adds weight, it can also damage the thin wall of the pole.
But why use a pulley at all?
A CASE FOR "NO PULLEY":
A pulley is often used with a metal mast to enable easy raising and lowering of the wire antenna. This is necessary because it is so difficult to raise and lower the mast.
With a fiberglass pole such as the Spiderbeam 12m telescoping fiberglass pole, the pole itself can be raised and lowered by just one person within 2 minutes, or 5 minutes when using the accessory clamp set. As such, a pulley brings little added value.
It is important to have the base of the fiberglass pole well secured, for instance as shown here,
and to use a 7mm nut driver instead of a screw driver for tightening or loosening the bolts on the accessory clamps. This combination enables raising and lowering the pole quickly as described above.
Assuming this is something you must only do a few times during initial adjustment, why even bother with the extra cost, weight and work of using a pulley?