For Platypus gravity filtration accessories:

http://www.platypushydration.com/accessories.asp

Sweetwater inline silt stopper:

http://www.campmor.com/webapp/wcs/stores...productId=13620

Seychelle inline filter:

http://www.backpackinglight.com/cgi-bin/backpackinglight/seychelle_in_line_filter.html

You're going to have to do some fancy footwork to make a non-collapsible pop bottle work in a gravity feed system.

Bear
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No fire, no steel.