Erik of Homegrown Evolution make a kayak balance stool. It looks fun!
Today I canceled my YMCA membership and started to put together my own home gym. Bored with the usual gym accouterments, I've set out to build some fitness equipment on my own starting with a kayak balance stool.
I discovered this idea in Christopher Cunningham's book Building the Greenland Kayak. To make your kayak balance stool, find a piece of scarp wood. I used a 2 x 8 and cut it to fit my ass to toe dimensions. Cut two end boards, each a foot long. Attach the end boards to the sittin' board with some bolts or sturdy screws. The deeper the curve on the bottom of the end boards, the more tippy it gets. Cunningham suggests a depth of 1 1/2 inches to start. I'd suggest making that curve a bit on the "pointy" side, as any flatness will lead to a lack of tippitude.