I've used the mounts twice: first time out, I fabbed a holder out of heavy guage sheet metal, log enough to have the plate follow the contour of the end of the fender. First ride in the woods, the sheet metal had bent "down", so I bent it back up. 10 minutes later, I lost the plate for good! lucky for me, the registry took pity on me and gave me a new registration.
2nd generation: I found a stainless steel plate mount (old Baja Designs) for an XR250, for cheap on **bay. Same basic idea I had, but much stronger SS. redrilled the holes, it's been perfect.
You ban easily fab something up, just make sure it won't interfere with the wheel travel, as the tire will fling that sucker far away! I belive I have 4 bolts holding it on, may be overkill, but after losing one plate, i didn't want to repeat. Plus, I know where I can find a spare bolt out on the trail if needed.
As far as the size, nothing special. I think it's the same size metric that hold the radiator shrouds on. Try one from there for fit.