+1 on the PB mount kit, it's well worth the $5. I had to fabricate a bracket to attach it to an old Blackburn rack with no rear mounting plate, I just used a piece of scrap steel folded over the rear of the rack.

I have a Cannondale rack on my tandem and it has a horizontal plate in the rear. I use the built-in clip and a tiny bungee to keep it from bouncing off.