Well, what a waste of an afternoon that was.
I tried peter's suggestion of some angle iron off one of the u-clamps. I went for the lower one, as there was still plenty of thread on the clamps to go through the extra metal.
Good idea, but doesn't really work. The rod or metal that you use as the third stay needs so many bends to clear stuff it's not funny - I got two attempts wrong by getting the bends either in the wrong place or to the wrong angles, so I gave up on that idea.
I have also tried taking a stay off the back of the upright, as the angle of travel from lock to lock is less than that if you had it on the hub side. This seemed to have some promise, as the bends weren't so complicated.
I got it up under the wing, but as soon as it was there, it was obvious that it was going to be a useful as a ashtray on a motorbike. It flexed just as much as stand still as it did without, so I can't see it being any good at speed, either.
So, unless someone can come up with a better idea, I'm just going to stick with the two stays and live with the wobbles - there doesn't seem to be any evidence of impending failure, and it's as rock solid on the upright as it was when I put it there.
C'est la vie...
