I have an early generation ACB Doubler in rose brass. It's much like a Yamaha 631. I also have Dupuytren's contracture of my left pinky, which is now bent to 90°.
I had been holding the horn so that I could could use my pinky and ring finger together to work the 3rd valve slide. But it finally got to the point that it was painful for the pinky, and the ring finger alone couldn't work the trigger very well.
Changing the grip to use the middle finger with the index or ring fingers didn't work for me; either the trigger wasn't long enough or the fingers got tangled up. I'd long thought that a longer trigger might be a solution, but couldn't figure out how to mechanize it.
The other day, I cut off about 2 inches of half-inch copper pipe. It would fit over the existing trigger, but wobbled all over and wouldn't stay put. So I cut about 1 inch of a half-inch wooden dowel and sanded it down to the point where I could slide it up into the copper pipe from the bottom.
The result fits snugly with no wobble and yet it is removable for cleaning. I compared the result to the trigger on a Getzen 4985, and I think it's a reasonably good functional match. If I hold the horn with one finger over the exit pipe, I can curl the useless pinky out of the way and operate the trigger with the middle and ring fingers. My hands are small, but even so, I can both fully release the trigger and fully engage it.
Bonus points for the fact that, with a bit of polish, the copper pipe will nearly match the rose brass.
I'd share this on the oTHer site, but they're too judgemental. 
