So if a 12 x1 nut is available, for the spindle, to get started, a bit of M6 studding and an M6 nut, would start the ball rolling, to make a proper stud.and nut combination.
For a "bodge" Tee nut, turn a bit of steel to 12 or 10 mm diameter, to the right thickness for the bottom of the slot.
File a couple of flats to give 8 mm, drill 5 mm, tap M6, et Voila! you are up and running and ready to make whatever "proper" bits you need. Next job, repeat, but with a stem, say 7.5mm dia?
Starting to sound like "There's a hole in my bucket, dear Liza, a hole" aren't I!
Possibly, a tee nut would be available commercially, if filing does not appeal.
Howard
Edited By Howard Lewis on 27/09/2019 18:55:18