I see no need for one. Just get an Ocean Rodeo Go-Joe or Skyway Scott's patented Blow-Joe (for more pleasure on the water) and the board will cruise down wind of you making it easy to retrieve and eliminating any worry about losing your board while relaunching.
http://www.oceanrodeo.com/gojoe.php
In my opinion, board leashes are like an archer attaching a rubber band to his body and then attaching it to the arrows before shooting them. I got hit in the head with my board while wearing a board leash in the race this year and even though I had a helmet on, it knocked me silly. I can't begin to imagine what would have happened if I hadn't been wearing a helmet. What if I had turned my head and gotten hit in the face?
I'll NEVER wear a board leash again while kiteboarding. EVER.