if you insist on doing this, go to a vehicle salvage yard and cut out a deployed airbag. the cutting will not be easy, you'll want a box cutter or other very tough cutting implement. sew a bag out of the airbag fabric. somebody else on this forum has done it, run a search.

unless you're in treeless desert, hanging (not counterbalancing) is easy. have you tried the PCT method?

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