Yeah, that is actually what has prevented me from putting one together thus far. The only "tough" thread I have at the moment is the orange thread that is used for jeans. That thread is way better than the regular thread, but it's no Kevlar. Plus, my regular sewing kit isn't large enough to cannibalize into smaller ones like my fishing kit.
I was thinking about taking those tiny spools and using them to hold the fishing line in the fishing kits though. Currently I'm using a foam ear plug.
Speaking of the fishing kits, about how much line do people suggest having? I was thinking that just a lead tied to paracord would probably be enough in a pinch, but I'm a pretty bad fisherman, so I don't really know. I think I have about 10 yards in each kit right now.
Two kits in each bag. One very small one with just hooks, weights, those little junction things for leads, and and a weighted jig and a plastic worm inside a plastic fuse box. One store bought Trout Kit with a few large metal lures and all the same other stuff as the small kits. The small kits go in the Altoid tins that can be carried on their own, or in individual packs that may be carried separately. Kind of a one is none, two is one, kind of thing.
EDIT: After handling this 10lb braided fishing line, I think this stuff is pretty dang strong. The $7.50 for 110 yards of this, compared to $20 for 65 yards of Kevlar is pretty appealing. Just noticed that you recommended fishing line, and not the Kevlar thread. Most of the time I've broken needles has been when I'm using a thimble. Sewing jeans is the most tough thing I've done. Usually break a needle when going through the seam where they are most thick. Either break the needle in half bending it too much, OR the eye breaks trying to pull thread through. The best part about learning to sew, is when you find out how insanely easy it is to sew super strong straps onto things. Seat belt + $2.00 sewing kit = super strong straps for whatever you need.