Grounding oil temp probe

I have an extension and 90 degree oil filter adapter on my IO-540 and am getting abnormal oil temps. (Too low with a small rise when flying.) I replaced the probe and have the same problem. I'm thinking its a grounding issue.
What is the best way to establish a ground from the oil filter adapter (or oil temp probe) and the engine? Should I run a wire from an adapter bolt to the engine? But which part of the engine? Thanks.

I would recommend a hose clamp or ring terminal on the probe shaft.

Used a hose clamp on round part of probe and ran wire to bolt holding standby alternator on accessory pad. Works great now! Could not get a ring terminal to work as there is nothing on probe for it to tighten against. Thanks for the suggestion.