My first choice would be to fill it with tinder (tinderquik, cotton ball, etc). My second choice would be an extra MP-1 tab or 2. These are things that I can't really see having too much of so if I have a an extra square 0.25"-0.5" then that's probably what I would stick in it. If you arent already carrying a small light then you might want to consider a Photon microlight also.