My Jupiter alto comes with one of those hard adjustable Yamaha style thumb grips, the ones that seem to curve the wrong way for a human thumb. Even though it's adjustable, and my hands are large, it would always leave me with a painful thumb.
This just slots over the thumb rest like a sock. It's thick and soft enough to ease the grip, without adding any uncertainty. It is smooth but not shiny and doesn't feel too sticky or slippery. It grips surprisingly well without any added glue, even on small metal thumb rests. My vintage tenor feels much more secure in my hand now.
It fits well on Midi wind instruments, Yamaha style modern saxes and tiny vintage fixed thumb hooks. I haven't had to modify it but you could cut it down to fit if required.