I realize that this is an old thread, but I wanted this information here for others to come across. It's a fine idea! Couple issues, though. First, Ursacks aren't made of kevlar or vectran! (I believe they've stopped using vectran, and if they used kevlar it's been years.) However, Ursack Minors are kevlar, but not for bears.
Next point is: it seems car air bags are nylon 66, not Kevlar (used for lifting air bags) or Vectran (for the Mars rover air bags). According to this document, this abstract, and this post.
Okay, so who's got the bright idea on cheap sources of spectra, vectran, and kevlar..? :]