Good point on the insulation.
That will be needed on my implementation between the sphere and bracket if they are touching, or I just make my bit of PVC pipe a fraction longer so that there is no direct contact between sphere and fixing bracket.
It does not though get round the fact that the copper sphere is attached with a metal thread, so that would mean using a plastic screw thread.
I’ve also realised that my cavity fixings are not going to be outdoor rated so i may just have to use a threaded stud and lock nut.
Potential solution to holding the sensor in the center found at the local tip yesterday…and old UHF/VHF antenna made of extruded aluminium rod … just the correct OD for a cable gland and ID for the sensor. Not ideal because its not plastic.