Ripstop sounds good but the sewing would be a problem for me as I don't have a sewing machine. My hand sewing is pretty rudimentary and whilst I've done it for work that doesn't have to be either accurate or pretty I wouldn't like to try it out on sails unless I get lessons first.
I found this morning that someone else had asked a similar question to mine earlier and one of the respondents suggested making sails out of the same material as kite making and gave a link to a website http://www.kitebuilder.com, so I will also look at that. I think Mylar is used a lot in kite making, which has been cited in my replies.
Nigel states that Ripstop has one side looking duller than the other and to keep it inside the seam. Does that mean you would recommend folding a sheet of Ripstop to make a sail double-sided?
Also what are people's opinion about the luff edge of the mainsail, should it be wrapped around the mast or not? From observing Ray's model above, Wild Duck, the mainsail appears to be attached to the mast with wires. Personally I prefer it that way but obviously wrapping it around the mast works too.