If you are sure it's silnylon, you can make a patch for it.
Find some matching cloth (if it matters) and cut a piece that overlaps the cut 1/2" on all sides. Clean/degrease the area with acetone and don't touch it with your fingers. Using clear GE Silicone II mix 1:1 with paint thinner (i use camp gas too but paint thinner is probably safer). Mix really well! Apply with a paint brush to the cut area , and apply the patch. Once it's set, you can re-apply the thinned silicone again to the top and bottom of the patched area. The edges will be sealed and you should be good to go. You can seal all the seams with this stuff and it dries in a nice clear film, almost unnoticeable. I've gone 2:1 thinner to silicone too, with really good results.
Make sure your repaired place doesn't touch anything else on the shelter until cured ...it's a permanent bond! Dust some talcum powder on it after it's all cured.
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