I have some of the big ones from ferrocerium.com, and am having problems with them once I get past the surface and start scraping the interior. I'll do more testing, and post it when I have some quantifiable results.

In the meantime, the Swedish Army large ones have been good, as have the ones they use in Strike Force's and Blast Matches. If you don't like all that plastic, you can always remove the rod and re-handle them.