I think I've found the perfect...

...window covering material.

(source)

Yes! I'm talking about burlap.  I did google this idea and I am not the first to have this particular lightbulb moment.

Here is a fancier burlap curtain:
http://patinagreen.typepad.com/patinagreen/2009/03/burlap-curtains-for-sale.html

Here is a no-sew burlap curtain:
http://vanillalavender.blogspot.com/2009/05/no-sewing-curtains.html

And more burlap ideas...the last curtains on this page will be what my lightbulb most closely resembles:
http://asoftplace.net/2009/03/linky-love-the-burlap-edition/

 I already have one "you're crazy!" vote. What do you think?