buy or find some suitable porous weave canvas-type material for patches (old boat tarp, etc.)
squirt this "tarp mender" stuff all over the area of the shelter's tarp and the patch
clamp both pieces together for 24 hours with wood blocks or whatever
flexible, tough, durable fix
both pieces need to be free of waterproofer and stuff so the mender can flow into the micro-pores of each material
later you can use the leftovers to make kick-butt sandbags
cut some jean legs off, pour in some sand, slather on Val-A mender on the ends and fold ends over like you are wrapping up a gift. The end result is plump happy sandbags with soft pliable glued seams.
I made some 15 yrs ago and they are still like the day they were made.
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you can also cut straps from jean material and slap them on the bags for easy carrying