I prepared to use what was to hand; a tennis ball and some bolts and brackets.
The trained monkey following a banana on a stick will come later.
Step 1: Step 1
A tennis ball
A 6mm nut and bolt
A strong strip of metal (this is a stabiliser bracket)
A bolt long enough to go through a tennis ball and two nuts (I used a 10mm bolt)
Step 2: Step 2
Step 3: Step 3
I used an 8mm drill.
Step 4: Step 4
I heated up a spoke and cleaned the hole up by melting the rough edges away.
Instead of any drilling I imagine that a soldering Iron could be used but it would smell a bit.
Step 5: Step 5
The nuts are tightened against the bracket and act as locking nuts, allowing the ball to still spin freely.
The excess bolt is then cut off and the burr filed down.
Step 6: Step 6
I fixed the whole ball assembly to this hole; the bracket was pre-bent to rest against the wheel with sufficient pressure to turn with the wheel.