After a few days pondering I came up with this

The harness is similar to the one I made for my rucksack strap but has a loop on the bottom and is fixed to the light mount on the seatpack using a molle clip - I bought some years ago and used to fix an additional bag to my handlebar harness using them.

The secondary fixing of the side release buckle is attached to the strap that goes over the seat rails. Even if the clip opened (and I've not had one open accidentally yet) the buckle keeps the SPOT attached to the bike. As a bonus the buckle actually stops the clip opening anyway

The component parts - some webbing, some velcro and a buckle

Whilst I had the machine out I made some additions to the bar bag recently bought from redefined_cycles

Two extra straps so it fixes to my Jones bars in the same way as my myog harness does and also some little rings to fix to the bottom of my feed bags to stop them moving.
I've also made a strap to stop the seatpack post mounting from working its way down the seatpost. When I used it on a recent trip it moved just enough for the seatpack to rub on my light.
