I've been using Berkley Gorilla Braid fishing line (20lb test) for high-strength sewing applications. Among other things, I've used it without any problems to repair backpacks, Army duffel bags (the huge OD green ones), my all-purpose carry-all, and even one of the ABS plastic foot straps on my snowboard binding that cracked in half on a really cold day on the slopes.
For me, Gorilla Braid holds knots fairly well as long as I tighten them slowly and leave a decent tag end. I use 20 lb test because it just fits through the eyes of the needles I use.
If you don't like Spiderwire and can't find Kevlar or Gorilla Braid, just try experimenting with these newfangled non-monofilament fishing lines. Find a brand and test rating (thickness) that suits your uses. I imagine that even though they're made of similar materials, the brand and thickness go a long way on determining how they handle.
If you can't find a fishing store where you can play with the different lines, just find some fishing fanatics and ask if you can see and handle the lines they have. If they're nice folks, they probably wouldn't mind just giving you a few feet to try out.
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Danny
aka "kuma575"