Not in any way presenting this as being best practice, and this set-up has only done about 75 miles or so, so I don't know how it falls over yet...
It looks like I've got a 25mm bolt going through the arms of what I assume is a Phaart mount from the spares box and a stack of washers thusly:
Washer action going on at the other end, too:
The prongs of the mount are splayed a fair bit to accommodate the light mount, so you've got a bit of wriggle room with widths.
I've a similar (shorter length) mount on the hybrid - I'll try and remember to look at it later when I fix a p*ncture.
