steven antonucci
They sell them in craft stores to make "friendship bracelets". It is a disk of stiff foam rubber with slots cut in it to hold tension. The Marudai can not only work with larger cordage, but works much more quickly, especially if you are ambidexterous (I am, moderately)

I started by making my own kumi disk (no shock there, right? ) out of a piece of craft foam (I wasn't spending $12 on one... )

steve