While I had the Thermal Core out and everything disconnected, I drilled a mounting hole in the tombstone of epoxy around the thermocouple bead, hand-twisting a small drill gripped in a pin vise.
That makes mounting the thermocouple much easier when the MK5 head gets tucked in place inside the Thing-O-Matic case. The washer is smaller than I’d used before, too. There’s no thermal compound under the brick, but I’ll probably add some the next time it comes out.
I pushed the insulating blanket back around the thermocouple and wire, then added a fuzzy button (punched out for the nozzle) atop the mess and taped it all in place. The thermocouple certainly runs a bit cooler than the Thermal Core, but I have no way of measuring the difference.
In any event, I think consistency is more important than absolute accuracy, because you’re tuning the whole affair for best printing at a given temperature, rather than picking an absolute temperature and adjusting everything else to suit.
It’s worth noting that the J-B Industro Weld epoxy in that block was in fine shape, despite roasting at nearly its maximum rated temperature for a few tens of hours. That’s not a lifetime test, but it’s encouraging.
The Smell of Molten Projects in the Morning 
Warning: JBWeld *will* fail on you. It is only a question of when. This was one of the very first materials we tried to use as a thermal insulator / attachment device. It fails every single time. Sometimes it doesn’t fail until 20-30 hours, but it DOES fail. It will crumble away and most likely your thermocouple will pop out. The only safe way to attach that is metal on metal. If you want some insulation, you can use kapton. That stuff is very rugged and will last hundreds of hours at temp.
Yeah, I fully expect it to crap out after a while, which was the motivation for getting that washer firmly atop the slab: at least it’ll hold the rubble together until the next time I peek in. I put a Kapton wrap around it the last time, but left that off through sheer laziness.
The trouble with using Kapton tape is that firm pressure from the washer shoves the bead right through; I know of two other folks who’ve had that happen in addition to me. The tape’s tough, but it does puncture, which means you can’t depend on it to keep the thermocouple away from a failed heater.
If I could figure out how to fixture it, I’d be tempted to drill a blind hole along a length of that red PTFE sleeve left over from the Riser Tube, cut the tube in half, and mash the section flat under a metal clamp of some sort. It’d both thermally and electrically insulate the thermocouple forever more. Probably too slippery to depend on, though.
The right solution, of course, looks a lot like a commercial screw-mount thermocouple stud, but …