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I've never had one that was exactly right. I've ended up with my thumb on the second slide pull ring and scrambling for the 5th valve when I need it.
I once saw a tuba that had a ring base that pivoted at the pad on the bell and had had an adjustable slide-rod, and then pivoted again on the rod. . . All in two dimensions, but it covered a base of many square inches. One could then, I think, bend the rod to adjust height too.
Is such a rig available, or would one have to make it?

When I went back to playing after a many year hiatus I realized that part of my problem was that with my thumb in the standard ring, my hand was in an awkward and uncomfortable position. I looked around and could not find anything like you describe in terms of an adjustable thumb ring, and in thinking of how to make one became concerned that the torque exerted on such an extension would probably constantly force it out of adjustment (or push it against the bugle and scratch and wear it rather constantly). However, I'd be very interested in seeing what (if anything) is available.

My own solution was to remove the thumb ring and then fabricate a little support bracket out of flat 1/2" wide aluminum strip which is attached to my second valve pull ring with a wing nut, and two stainless steel washers with rubber inner washers. This puts my hand in a much better (lower and better aligned) position for the valve set. It doesn't look real classy, but it works and only took be a bit of experimentation over less than an hour to do.

Since I never adjust the second valve tuning slide (if anything, my horn plays a bit flat on the second valve), the support stays in the same position all the time.

I was having a similar problem - my horn didn't come with a thumb ring (ring was just missing but the mounting was still there) and I was instructed to find one by my teacher... apparently my hand was flopping around a bit and needed stability. When I got it and screwed it in, I found an angle good for the four fingers but I couldn't comfortably reach the 5th valve with my thumb. Solution: Dana Hofer soldered a quarter, at an angle, onto the thumb lever that simultaneously extended it slightly and also allowed me to comfortably roll my thumb over it. Great solution = simple, inexpensive, easy, and effective.

Never saw or used a thumb ring that wasn't WRONG! This includes my 5J. Much prefer the tubing loop behind the valve block that you see on many Euphs. Firm enough in the fast stuff and infinitely adjustable, at least laterally...