Or WTF did I do wrong??

I decided to relocate my Cherrybomb from the seat post to the left rear rack leg. I grabbed an old innertube, cut it to break the 'circle', then proceeded to wrap the rack leg with the tube until it was roughly the diameter of the seat post, clamped the mount around the tube, mounted the light... and the whole assembly rotates freely left to right . I then got the bright idea to attempt to kill two birds with one stone- I ran a zip tie through the mounting clip and around the rack leg. It still swivels. At least the light won't be going MIA on me. I've now got a rubber band that is acting like a spring between the mount and the fender strut.

I really want to make this work...

When you say the "whole assembly", are you saying the inner tube is rotating against the rack arm, or the light mount is rotating against the inner tube?

I've not had great luck with using inner tubes for those kind of applications. It seems they have some kind of powder inside that makes them kind of slippery even after they are washed off. The only time I have had luck was by wrapping quiite a bit of electrical tape around the inner tube.

It may be there's just too much of a thickness to work correctly. Doesn't it mount to the back of the rack?

I have two lights. The Cherrybomb has solid and flash modes and has a seat post mount. I have a B&M 4D Toplight that doesn't have a flash mode mounted to the rack.

I have a rack trunk that I use sometimes, but it either partially blocks or totally blocks the Cherrybomb depending on how full the bag is. I will have the same problem when I switch back to my 'flatbed mode'.

And I just double checked- the mount is secure to the tube, but it's the tube that is rotating on the rack leg.

Do I need to use a bit of tape of some kind to offer some friction between the tube and the rack leg?

That's pretty much the same results I had. Those inner tubes are slicker than you think.

You may try wrapping electrical tape around the rack arm before wrapping the tube....maybe that would give it something to friction grip. That's all I got unfortunately.

Might I suggest mounting it low, using your left rear rack strut eyelet. Looking at a photo of the CherryBomb it looks as
if it uses a clip to attach it to the seat post clamp. Perhaps with a little judicious cutting/ trimming of the clip assembly
and with a single short seat stay rack strut bent and trimmed to the proper length, you could fashion a short support
to hold the CherryBomb secure.

I know... you're probably looking for a simpler solution; it's just the wanna be fabricator/ mad hatter designer in me.

I can kind of see the design in my head, though...

Alan

Start with your strip of tube and a piece of electric tape stick the tape lengthwise to the tube with a few inches hanging out. Start the tape to the post first and that will fix the inside end as you wind it. Wind it tightly so the tube material stretches as you go. When you get slightly larger than the seat post size finish the end with another piece of the tape and a couple turns. Apply clamp and you will be good to go.

The tape+tube+tape again seems to be the solution. There's just enough play for alignment adjustments, but pretty snug otherwise. Thanks!