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

can you post a zoomed out view of it on the bike? im having trouble imagining it as a whole.

Nice fabrication.

Sure, again sorry for the quality. You can see it on the right side of the rack (now with blue zip ties on it!)

No need to go through all that. Just wrap the O-ring around a rack leg twice. Like this:


Works on the outside of the rack leg...



...or on the inside of the leg to clear your panniers.



Battery pack velcros to a rack crossmember and rests on the fender.

I tried TSL's method, but it didn't work with my rack. I was able to use the o-ring to hold it to the bottom of the rack. That works on two of my bikes, and is easy enough to move between the two.

TSL I tried that method but found that it angled the light too low for me, I like it aimed a bit higher - using the original seatpost mount gives me the vertical alignment.