I’ve got some old QR wheels which have 15mm ID bearings in them, with a spacer between the bearings so they go into a 15mm fork with a little bit of faffing getting the inner spacer lined up.

Would plastic end caps work? They’re not a moving part, and they’re not hugely loaded?

All they do is control end float so they will be in compression – you won’t die but they might squish a bit and leave a wobbly wheel

Be interesting to see how much they squish by. Suppose at least if they do I’ll have the right CAD file to get some machined!

Or I could find some 15mmID tube machined to length and just 3d print the seal part of the end cap…..

I reckon you will be ok, whilst waiting for my new wheeels, i used some plastic tubing to form spacers so i could run my non boost wheels in boost forks. It worked out fne, just a bit of a faff puttng the wheel on and off.

Print in wax and have a go at casting end caps?

(How hard can it be?)

Investment casting. When my dad trained as a dentist in the early 1950s they had to produce a cast of s fly.

A friend of mine recently took a pair of Hope end caps and cut them down to fit a set of Alex hubs.

I 3D printed* some adjusters for a 130mm disc hub to allow it to run at 135mm. They worked fine and still do, I believe.

* the were made to aerospace standards and from aerospace aluminium as that’s when was being printed at the time, so that’s what they got smuggled into.

I did some to fit a 15 front and 12 rear in a qr wheel truing jig, used PLA and it was pretty tough, can’t see it squishing myself, cracking and breaking maybe but I think its unlikely.