"The bottom insert needs to be made. This would be a shape extruded up from the zero point to 15mm, then Boolean diffed out of the part (don't forget to delete the 2 solids in the cavity after this)"

Huh? You are talking about the 15mm tall "plug" that fills the bottom end, and contains the tapered hole? I think I get the Boolean, but what two solids in the cavity?

Nice job on the insert, but that's upside-down - the narrow end goes into the stepped and tapered hole that they call out, at 250C, and the ridges on the hot brass insert push melted plastic forward into the serrations and ridges as it slides in, for a really solid permanent embed. The hole needs to be wide end at the bottom, and shaped like the attachment. The 9mm figure was just a guess before I spotted the hole shape at McMaster.

Tapered Hole Diameter, mm
(A) 8.2
(B) 9.22
I'd put the narrow straight part of the stepped hole at about 2/3 of the way in, guessing from the sketch. The straight part of the hole needs to go all the way through the plug to the interior, of course.

Warm thanks once again, for your incredible work and sharing how you did it.