I think based on the comments the standard hobie 20 sail sheeted to the rear beam is just not as fast as a modern high aspect spin. The F18 is too small and I assume no one is suggesting the I20 spin is the H20 has a comptip or shorter mast.
I would try to find someone who has already had one made. If you cant find one, you have to go custom. The good news is you will save money; the bad news is that it is it will take some tweaking. It is worth it though. Start with reading this article.
http://www.geocities.com/kustzeilen/Genaker/GenakerWord97.zipIt will give you some info on how to rig the spin. For example, you can use Amsteel line for the spin pole bridle instead of having a metal one made.
Next you need to find a sail maker, here are some I considered and a quick comment on what I found:
Calvert Sails (much more expensive than the other quotes I got, they must feel that they have a significant advantage)
Ulman USA (mid range price, great sail shapes)
Landenburger USA (mid range price, great sail shapes)
Wirlwind Sails (low price, they use landenburger mold shapes for their sails)
Good luck,
Matt
