Oh Shinola!

Magicshine. I have one of those too.

What I did was cut an inch-and-a-half length of brake cable housing to use as a shim. I mount the light to the seatpost, then pull it away a bit to wedge the chunk of housing between it and the seatpost at the level of the lower pass of the O-ring. It's just the right size for my seat tube angle and Thomson setback seatpost.

Alternatively, a piece of one-inch PVC pipe could work. Use a spare O-ring to attach the PVC to the seatpost, then attach the light to the PVC. As a bonus, you can use the PVC as a place to stuff extra power cable.

That's what I was going to try, anyway, before I discovered the chunk of housing would work and do it more simply.