Definitely wasn't thinking of sharing a single supply - more like dedicating separate supply to the heater. They cost less than $20.

Peeling the white stuff from the base build plate might not be wise. I suspect there was a serious 'flattening' process when they made the plate. I suspect you will need to flatten the plate again using a granite stone with sandpaper. Can be done but you will want to check flatness of the backside. I know they are not very flat but what is tolerable?

Can we design the traces so that the magnets are 'reverse' heated? Say - leave sufficient annular clearance around the magnets so the aluminum around the magnets are somewhat cooler than the remainder of the plate. Let the heat come from the surface, so to speak.

What temperature are we aiming for? And at what temperature do the plastic caps start to deform? Should it all be metal on metal; magnet directly to the plate, caps removed? The small pillars that hold the current magnets are a great heat-break to the carriage.

How about glass with magnets? Would that make a better surface, w/ or w/o LokBuild? I still have a great 170mm B-glass plate that is absolutely dead flat. That might be a good place to start with know flatness. This also provides [and robs] an 1/8" for the magnets and gives a little breathing room for the carriage. Worth considering.

I'll look into the thermistor. I have no clue as to software options so I need guidance there. I just need to be able to see it to solder it.

5 amps isn't hard to deal with. I'm very tempted to just bolt a lug on but no. It requires very robust strain reliefs on the wires. 16awg silicone wire should suffice for lead-wires. A pigtail connector isn't out of the realm of possibility either. How about a 5.5x2.1 setup for quick disconnect? Very DIY compatible.

Adhesion is the only way I can see keeping the PCB close to the substrate being heated.

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Where did you want the heat to be sensed? Yes, they make good quality thermistors that are surface mount. an 0805 package can be soldered with relative ease. Just need to know what I am looking for as value goes.

Considering the price of the PCBs are cheap and most of the cost is shipping, make a 'heat-shield' board. All copper to reflect heat back up and avoid heating the carriage components. These simple techniques are used on cars very day. A simple radiation shield is highly effective.