Thanks for your responses.
Not keen on the plastic bung but quite like the idea of the spike.

 

Here's another idea... More details on my web site.

 

Nice pipes ...

 

Harley sells a gnurled thumbscrew bolt for use with the seat in this hole. . . I bet it's not too long to use in the hole without the seat bracket. It has a logo on top, nice looking.

 

You could also consider a luggage rack?