There are various shorting adaptors available on the internet for purchase, but I found that I could make one using a metal drawer handle. The standard threaded screw used for attaching these is roughly the same diameter and pitch as the thread at the top of spark plugs.
This is the drawer handle with a plug screwed in instead of the normal mounting screw.
With the posts unscrewed from the handle. The screw through the middle has had its head cut off.
Installed on the bike. The exposed metal allows shorting to the cooling fins.
16 January 2024: An update on this: I realised eventually that the thread is the standard M4, so some long nuts and bolts and couplers are all that is necessary:
