I had the same problem when I initially set up my chrome-top starter box as well.

You need to keep that shaft from moving while you tighten down that nut. What I did was not the best way to do it, but I stuck a screwdriver into one of the slots of the motor case to keep the armature from turning while I cranked down on the nut.

Now that I think about it, you can probably try finding a few washers to stack between the nut and starter wheel so it gets tight sooner. Once it gets so tight, you can hold onto the rubber wheel and tighten the nut.