1982 KZ1300 rescue and rebuild

It would need to be very heat resistant
No one has ever taken an I/R temp. reading of "a" resistor-block?
I'll put that on a to-do list as well.
If there's concern (and it appears there is) - I'd suggest - rather then cover the resistor block, perhaps consider adhering something thin to the tank in the appropriate areas.
Better cooling by air-flow is the obvious advantage.

The tank may sit on "Rubbers" but it is grounded by the Fuel-Gauge ground-wire.

That was my thought as well. I just checked the continuity and the sensor bracket shows no resistance. So the tank would be grounded by the mounting bolts.

perhaps consider adhering something thin to the tank in the appropriate areas.

That was also exactly my thought.

The mounting bolt shouldn't be considered as a "ground" because (on mine) the large grommet insulates the bolt from the frame !

ANd that's what I thought as well, hahaha! The whole thing sits on an o-ring. But! If the body of the sensor is 'ground', the bolt touches it when mounted, and bolts screws into the tank, that would make the tank ground as well, right?

Nice, neat wiring on the examples in the photos ! For others considering this option I'll screw things up with this suggestion:
Keep the OEM "Single" power feed to resistors and make a "Buss-Bar" that connects the 3 resistor inputs. Then attach the single power IN lead to any one (1) of the "Bussed" screws.
A crude illustration !

Not really enlightening wonderful news of technical nature, haha - just to document and a follow up on the 'three bolts only' on the handlebar question.
Now all is fixed.