Most people who study tea ceremony that I talk to say that sitting in the seiza position is the biggest challenge they have in studying tea. I have found that the seiza isu stool helps a lot. It takes some of the weight off my lower legs, but still gives me a good position from which to make tea. Try one to see if it helps you!
They come in two sizes: Large, 12 x 20 x 12H cm; Small, 10.5 x 17 x 12H cm, and in two colors: Blue and Red (reddish orange).