Hi Jules, does it have to be mobile?
If not,
this thread has some photos and plans for the one we had made when we were getting our fences re-done. The best thing about it is the internal 10' gate, which functions in much the same way as MF's trombone / forcing gate. Animals can then be fairly easily moved into the small square pen for treatment.
The worst thing about it is not having an actual race (I'd really like one of those, and may well install one soon in place of 16' gate on the right hand side).
The other thing that didn't work well was the stones.......
We got them basically for free (long story), but they were too big, and the ewes didn't like walking over them, particularly if they ever had sore feet.... and of course that's exactly when you want to catch them! So, I dug them out last year and put them in the bottom of a soakaway, but then I made the mistake of putting in 10mm gravel instead. In hindsight, that wasfar too small, and can get wedged between clees or in hoof pockets. We also put a load on our drive, and are forever walking it into the house. I sometimes wonder if my sole function in life is to act as a warning to others, you know
.