why would you want to "reseal" it in the field ???
I do use this system. If I need to use one, I cut it open and that's it : I will use all the coton there is in that piece of straw. Maybe I will even open another one if it looks like one is not enough tinder.

Granted : it's a bit of work to prepare those. But they are easy to store anywhere and they are waterproof.