I too am looking for an adjustable thumb rest but for a different reason. Not to relieve pain. I am 78 years old and learned of the rubber tubing solution for that when I was a youngster taking lessons from an old Sousa clarinetest.
My problem is that arthritus is developing and the index fingers are bending toward and under the middle fingers. The left hand poses no problem but the right does. When I get the index finger in position to cover the ring/hole, the other two fingers are not in correct position. And of course the reverse is true. I have found that if I place the thumb above the thumb rest it works but of course I have lost the support of the thumb. Hence, I need an adjustable rest for a Buffet that will go up, not down. Any help you can give will be greatly appreciated.
I understand your problem in that I have one not unlike yours. There are two possible solutions: The first is easily possible by moving your thumbrest up which is fairly simple. This may ameliorate your positioning, however if not I recommend you get a thumbrest that will help to open your hand, bringing it away from the rest of your palm. The Ridenour Thumb saddle will do this . They are really inexpensive and they have helped me enormously, mostly I think because you get to change your position, any change being one for the better, at least for a while. I have about four or five of them and always keep them handy when pain sets in. Just look down at your hand(without clarinet)and then open your thumb away from it. You will notice that your fingers will open involuntarily. While you can keep them still, it is natural to open your hand a bit. Add the clarinet and I believe you will have a solution, or at least a help with your playing.
Good luck. ()