1/8"X 2" ferrocerium rod. Drill small hole (1/16") near one end (Ti coated drill bit). Insert small split ring through hole to attach rod to key ring. That size rod doesn't cost much more than the Zippo flints and it eliminates the worry of the flints coming loose from the key. And it's smaller since the key itself is eliminated. Just my .02. BTW cool the drill bit and ferrocerium rod with water frequently or it will spark like crazy.