I haven't posted for a while so thought it was about time I gave an update.
After much though, a couple of years ago I decided to build a Gary Dierking Wa'apa sailing canoe. I built it using Gary's excellent book Outrigger Sailing Canoes. Initially I built the 16 foot version. The sail is the fantastic Solway Dory 55 foot Bermudan rig:
I built the full sized amas and they do look a little long with the 16 foot configuration, however they perform very well and dampen any hobby horsing effects that can occur in shorter double ender canoes.
Since this initial build I have done a number of modifications including adding additional sails, and building a removable 5 foot centre section for better two person sailing. I'll add some more updates and some videos soon, but suffice to say I am very pleased with my car top sailing canoe!