Forgive the attempt at humor, but I'd be wary, in these days of fears of terrorists, of "defusing" light. I'd always be careful to "diffuse" it.
In answer to your question, I had one friend suggest using sailcloth (very strong, fairly light, resists moisture / mildew / rot / etc), but it's not cheap stuff. I priced a small amount and was rather shocked at how much they wanted for it. Still, it would probably do a very good job.
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