So, the pulley is glued in place with Pattex. I also have found a little demo sketch for my little stepper and wrapped a rubber band around both pulleys to see the thing spinning for the first time.

Sort of.

Not surprisingly, the whole thing does not move smoothly but does not move at all at first (thanks to the excess friction in my improvised ball bearing) and then wildly jiggles because of the excess flexibility of the rubber band.

So, new (printed and smoother) races, some kind of cage and a proper(er) belt are next...