As the previous axle (steel tube) was not straight enough, I am now using a 10mm threaded rod. This joins to the bike bolts at both ends, and as they have different threads, I had the two nuts of each welded together. One of the nuts also has a circular plate welded to it, which will support the aperture wheel. I assembled the parts and here is the effect:
Where I am holding the cogs is where I will have a wooden support/ frame that attaches to the lid. The freewheel clicking works really well and I like that it sounds like a cine projector.
However, I have realised that there is a big fault in this design. The freewheel will only allow the wheel to spin with the wind in an anticlockwise direction, but I need it to spin clockwise or else the nuts and bolts all try to unwind and spin from their various positions. This would be a disaster, the wheel spinning off and hitting someone.
My solution is that I will have to sacrifice the cool clicking noise and freewheel mechanism, and instead use a front wheel, which will also be much lighter and generally less strain on the mechanisms. Here is the threaded rod axle, inserted through the hub and tightened in place with nuts either side. Now my half-bucket wind vanes can be fitted onto this wheel so that the wind can only spin it clockwise.