Hi:

I used Spin Rite ( cheap ) polished aluminum spinner on my UCD.
It required enlarging to fit so you must balance it afterwards.

Even if you do not modify, you should still balance them as the machining is never perfect.

Tools needed.

Top Flight magnetic balancer.
hand drill with 1/8 bit
dremel with carbide cutter

step 1

balance the back plate only. Mark the heavy spot and put drill hole(s) on heavy side from inside the back plate but do not drill through. drill couple more on either side of the hole if you still can't balance. do this until the back plate will stop at any spot.

step 2

balance the cone with the just balanced back plate as a guide hub. ( without the back plate you can't center the cone properly)
mark the heavy spot and carve out some material from inside the cone. balance until the spinner will stop at any spot.

try this new balanced spinner on BALANCED prop. you will notice how smoother your engine is running.