After seeing an instructable for a bracket to hold a unicycle to a motorbike I thought that I would post pics of my solution to the same problem. I have included plenty of views so you can figure it out for yourself. Sadly I have no photos of it being made.
I came up with this bracket design after several iterations that held the unicycle in various places on the frame and wheel. This final design uses the end of the crank where the pedal screws in as the main holding section. A small elastic strap stops the crank from swinging forwards.
The pictures do not show it attached to a bike as I wrote my motorcycle off whilst simultaneously learning that a motorcycle crash is far more painful that a unicycle crash. When mounted to a motorcycle it bolts onto the mounting holes in the motorbike frame. The unicycle leans downwards and forwards and the seatpost is strapped to the pillion footpeg. The result is a bracket that is discreet and easy to attach the unicycle to. It will hold a variety of sized unicycles. I have used it from 20" up to 29" wheels.
Got any questions? Feel free to ask.