The lens isn't designed to accept one. The knurled area is too close to the body, then there's the switches/distance window, and the bare part after that rotates (it's part of the focus ring, just not rubberized).
As a generic (but expensive) solution for non-collared lenses, you could look at Really Right Stuff's Camera Rotation Device. I think Custom Brackets has something like it, as well.
EDIT: Regarding simply finding a collar with the right diameter, lenses designed to take a collar have a 'flat' area for it. There are highly customized (and costly) solutions for some lenses - well, actually I'm only aware of one, the Hartblei collar for TS-E 17/24 lenses to eliminate parallax for shift panos), not aware of anything for the 135L.