After taking some inspiration from setups on here this is my setup - done because I have a trunk bag which means seatpost mounting is out. Sorry for the relatively poor picture.

You will need:
A bit of tubing / handlebar / something cylindrical - I used a bit of handlebar.
Duct tape
Dinotte seatpost mount
Zip ties

I duct taped the handlebar cutoff to the rack. Then attached the seatpost mount in the usual way (around both the rack leg and the handlebar cutoff). Then used zip ties to make sure the seatpost mount won't slip off the handlebar cutoff (not pictured, but they go around the handlebar cutoff lengthways).

I did this because if I used the method of just double wrapping the o-ring around the rack leg the light was angled down a bit too much. This setup allows me to adjust the angle of the light.

Hope this helps someone,

Daven