Found this site on how to make a sail out of Tyvek. Kind of an interesting idea and it would be cheap.

Don Haas
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Making the Tyvek Sail http://www.boat-links.com/Tyvek/

Making the Tyvek Sail. By Bill Wallace. As you travel around near where you
live, keep your eyes open for a house being built. ... These too are made of
Tyvek. ...