OK great, thanks. And sorry it was you who mentioned the KF insulator, not roxy! Mis-attribution! 
Good point re the silicone, I will keep an eye on that. I just double checked and it's pretty firm - I had to tighten the screw very hard down onto the silicone + o-ring sandwich, and pushing on it now I can't notice any give at all. But that could change over time I suppose.
I'll try and do something better for the permanent solution when I get my longer screws. I've seen PEEK insulation sheets on eBay, maybe that'd be another option to consider.
Good point re the silicone, I will keep an eye on that. I just double checked and it's pretty firm - I had to tighten the screw very hard down onto the silicone + o-ring sandwich, and pushing on it now I can't notice any give at all. But that could change over time I suppose.
I'll try and do something better for the permanent solution when I get my longer screws. I've seen PEEK insulation sheets on eBay, maybe that'd be another option to consider.