Tough one this, I went with the retrofit guide offered by Rohloff itself with tie-wraps, and it su#@s. It allows way too much freeedom to the cables:
the gutter of the Ringle bottlecage is the perfect guide lower down.
Idworks bikes uses nice twin small guides that actually clamp each cable and fit with a bolt in the middle, but you need fittings in the frame to bolt onto which my bike does not have. With crud catcher mounts under the downtube you'd be in business.
I hope you 'll get the perfect reply so I can copy it
Common sense was never common
28h silver rear Suntour hub
32h Ritchey champagne WCS front hub