Hey MtlClimb ! The assembly is the same on both bikes; the lower section is 40.5 mm at the bottom tapering to 40.0 at the clamp, and the upper tube is 34.9 mm, which is the same as the seatpost diameter. Hope this helps.

I'd be interested to see what you end up going with, as I wouldn't mind some handy carrier dealies like that, myself.