Hi NZO,
I think what you want is possible as you say.
For the printing of the sails, i don;t think it wil matter if the belly faces up or downwards for the printer. Keep however in mind that the facing down part, wil be less prety due to the support structure. Just choose the side you would like to be nicest to be top.
For the thicknes, personally i would recomen making it around 3mm thick, probably it could be thinner, but with this thicknes, i think it will be quite rigid, and doesn't brakes easily.
In the designing, I unfortunately only have experience in (student) version of non-free CAD programms. I now it is quite easy to make them in Solidworks/Rhino orso.
Perhaps you could check if there are any existing sails on existing things like: https://www.thingiverse.com/thing:923580
And if it would be able to scale in x-y-z directins in Cura, to get you're dimensions?
(If you're really at wits i could help you design it)
Kinds/regards,
Ewoud