For my own reasons, I decided to remove my rear rack. Down side is I recently acquired a B&M 4D Toplight from a fellow BF user. I like the light, but it was designed to mount to the rack. I found this adapter on Peter White's site: .

I did email him and asked if it would work on my bike with its QR set-up, he wasn't entirely positive. I have a QR skewer like this.

Anyone know if the above adapter will work or have any better ideas?

Seatpost. You could probably fashion something sthat would work by visiting your hardware store.

I suspect a hose clamp and an aluminum bar (like is used across electrical junction boxes) might work. (Of course, you have the duck tape one to the other...)

I had a similar issue with mounting my B&M Toplight. I do have a rack, but it was not designed for mounting this sort of light. I considered mounting on my leather saddle, which has two tabs for a bag in the back, but worried about the stability of the light as well as the effect of me moving around on the saddle. I also thought about making, or adapting a seat post clamp to mount it but then my seat bag would present a bit of interference, requiring a change in bag or position. Since I was retaining the rack, I decided to make a mount out of 1/8" sheet aluminum and drilled/tapped a couple of holes in the back horizontal bar. This actually worked well, but it is still a prototype (the 4th one, that is). I still would like to figure out a way to mount it to the seat post, but at least I am covered for now.

I reckon the moral of the story is that you probably won't be happy with your first couple of trials, but it will work out.

Here's an example of what I was thinking for a seatpost solution:

https://www.amazon.com/Topeak-CageMou.../dp/B005HM9MXC