Backpack O2 Tank:
Large cardboard tube, (diameter of 2L soda bottle) or whatever you find that works.
Old Vacuum cleaner hose
1. Coverered tank with cheap $1 dollar store silver dash window screen
2. Took some fish netting and covered over the O2 mask. Connected hose to mask, and inserted hose in bottom of tank. (photo added)
3. Just be creative and futuristic!
Bought a used pair of welding goggles, painted them copper/gold/etc, added little extra pieces like a flash light.
Rest is self-explanatory, no reason you would need a full blown laid out instructable. Again, just google Post Apolyptic and get some ideas. Instructable has some things in re: to costumes to look at.