[18:20] I was worried about the proximity of the cartridge holes and the wall thickness of its adjacent thread, but the technician had no issues to report. These are both 3mm plates.

The coupler plate slot might be a bit more of a squish than I originally thought because I didn't account for the thickness of the cartridge wire protective sleeving, but it should still work fine. The cartridges fit through so I don't have to desolder and resolder anything just to install/remove them. 

I'm glad I had the foresight to even design the coupler plate instead of trying to make some 3D printed part that either held in M6 nuts or (if they even exist) heat inserts. Like the heatbreaks, there isn't a lot of thread on the PTFE couplers. This way, the end product looks better, the couplers are less likely to bend/break off and-- ohhhh I knew I was forgetting something. I've just realised that Me In The Past also wanted the plate to act as a stiffener for the printed holder. 

[18:50] Modelled some M3s in:

The top looks less sleek, but in practice you'd likely never see it: