With my limited glass knowledge....don't you have a decent template already? You have a support system on your jon right? Take that out, spread your fiber, and glass that sucker! You could use a nice thin sheet of luan plywood. It will bend with the curves, has almost no weight, and you can remove it later to put your decking back.
Just a thought, I may be wayy off base, as I've never actually glassed anything myself, only watched as friends built panels for my car
Vince