I just saw the pictures of your sails. Good job. But to improve the sailing qualities, it’s a must to have a little curve(camber) in your sails. Like I said they explain it on the website of www.MYRC.org (library). But the simplest method to obtain any curve, without the use of a camberboard etc, is to cut your luff and foot of your mainsail a little round. You will see that it improves the sail properties.
Especially when you’re using thinner sailfabric (like spinaker/kite fabric).
Tyvek is readily available from http://www.kitebuilder.com . Go to their website and loof under fabric. I orders some Tyvek FTV 14 yesterday at $3.25 a yard for 60" wide material and they’ve already shipped it. Good Luck!