Thatīs what I have now.

The collet set has 3/8 x 24tpi (UNF) threads, I made a drawbar for them, the rightmost one.
The extra collet has a 3/8 x 16 tpi (BSW) thread, the drawbar for it is in the middle. That drawbar also works with lots of other tools acquired from England, of course.
The original drawbar is M10 x 1 (fine) thread, on the left. It fits most of my stuff, drill chuck, ER25 collet chuck, etc.
Donīt you just love standardization...
All the threads are very easy fits in their resp. tools, "tuned" with split dies.
Thereīs also two brass washers in the middle drawbar, to adjust the length of any of them. I can fix any tool I have, easily, securely, but without unnecessary force - except the collets. I just canīt hold the collet tight with only 5 mm protruding from the quill, without doing awkward contortions.
Iīve been thinking about something like the tools used to extract oil filters, but in miniature. The collets are 18 mm dia. at the end, and of course, slightly tapered (M2). So thereīs not much for the band to grip on. But such a tool would have also the advantage that it would close the collet around the cutter, and prevent it from falling down when it is being tightened. Something like this.

That is for diameters from 50 mm up, so maybe not usable with 18 mm... But the idea seems worth experimenting. Is there something like that, but small, ready-made, or should I cobble one up myself?