Check out the plans for a double layer design here: http://www.backwoodsdaydreamer.com/diy-guides/hammock/
. I have used those plans for my first DIY and it turned out well. They are also a good source for fabrics and other supplies. Scott is a great guy to work with too.
You can make a single layer out of the 2.8 plain weave nylon he sells if you want to go that route and have a little more confidence in the capacity. I am a fan of double layers, though. I sort of have both with the Magna-Hammock.
Good luck, have fun, and hold on!