Mine is currently mounted to the eyelet at the bottom of the seat stay using a post mount to IS mount brake adapter bracket I had kicking around in the parts bin.
Thankfully my bit arrived from Frambuilder Works, 5000 miles away in less than a week. I'm sure there's a fabricator somewhere closer who could have made me one, ah well. : /
I've got a Supernova rack mount rear light as below, which I want to mount down by the RHS seat stay.
I've got this on the way from the US from Framebuilder Supply, taking its sweet time to get here:
Any tips on mounting this until the bracket gets here?
I mean I guess I can botch it with cable ties, but anything more elegant?