4.5mm BB thrust bearing

This is not a usable design, just a test. You'd need to punch an axle through it in openscad or such, or more likely just incorporate this wholly into your own designs.

Got a jug of 4.5mm airgun ammunition, seeing if they'll make usable bearings. Pleasantly surprised that it worked first try, the balls fit the races and the tolerances didn't seem out of whack.