While an Israeli Gas Mask and Filter usually runs less than $50.00 from any number of sources on the web, and a better bet than this, I thought this DIY "Gas Mask" was still pretty interesting.

The author got the idea from a several articles produced in 1942, one from Time Magazine, and another from Popular Science with illustrations and instructions from that article. The Instructables tutorial is worth reading to see the reproduction of the Popular Science article alone in my opinion.

The gas mask the author produced doesn't have eye protection, a drawback, but still...
DIY Gas Mask on Instructables

Enjoy!