I just ordered a double layer made of 1.1 nylon. It was suggested to me that anyone over 200 lbs should opt for 1.7 nylon due to the possibility of stretching. If you look at the chart at Hennessy's website you can glean some info on materials they use for hammocks rated for different body weights and save yourself some trial and error.

There is also lots of DIY info at hammockforums.net, on different styles for different purposes - great place to start researching tarp and insulation options as well.
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