Oops. I wrote my reply below before scrolling down to your post. You pretty much said it, but since I already posted it too...


I've got a Tubus Cosmo and a DiNotte 140L too.

I mount mine on the lower siderails where it bends down to become the rear leg. You just have to wrap the O-ring around twice.




I mount it on the inside of the tube so it doesn't interfere with panniers.




These days I use a B&M D'toplight dyno taillight on the Tubus' light bar. I like it because LED protrudes in the center pod. Makes me think of a Groucho nose. It can be seen a good ways away from the either side, which is the reason I chose it.




Before I got the dyno setup, I used a B&M 4D Toplight battery light. Four diodes--two pointing to the side and two in a protruding center pod. All B&M taillights mount directly to the Tubus rack with no adapters required.




Wayne at TheTouringStore.com carries the B&M battery-powered taillights. He's just down the road a piece from you in Loveland.