The shrink tubing is a good idea, I've found through experience that the ferrocerium (actually, I used 5 lighter flints) will come loose and disappear through the normal wear and tear of jangling around in one's pocket.

I'd use proper electrical shrink tubing, not shrink wrap, but I don't have anything other than a gut feeling to go on there. (That is, if you mean kitchen plastic shrink wrap, I think it would be more likely to come loose, snag on something, tear, etc. than shrink tubing. But maybe that's a language/dialect thing. )