When I purchased my Oyumaru, it came with a shell mold. I cast a few pieces in it and the idea came to do the fish painting into the shell.There was a problem with this, it was a full solid piece.At first, I was going to just sand the parts that I needed out then go back and do the layered painting. But sanding it turned out to be more work than I had planned on. After thinking on it for a few days, I finally found a solution. I tucked some Oyumaru into the cavity and then used that to create the shape I wanted.