Due to an accident several years back, I have limited use of both of my hands. I have lost considerable grip strength and suffer from frequent cramps to the point that I had to give up golf. I am relatively knew to carving/stamping, but not to leather work. I too suffer from cramps when working with my stamp. I have tried everything, from tape build up to waterhose pieces to now fuel line hose "doublewide." I got this idea from a older friend of mine who was a golfer, but he suffered from arthritis in both hands and had to have his grips built up. I took the same idea and use the fuel hose to build my stamps up.
I find myself having to take a lot of breaks and keep one of those squeeze balls at my table to stretch the hands, but then again, I also suffer from neck pain due to the same injury and haven't found a way to deal with this issue.
Good luck and happy carving!
ATX