I won't be a smart aleck and say you have two choices: Use a credit card for a new shelter half, or use needle and thread.
But in all seriousness, I recommend doing what I did with an expensive torn gum blanket. I did what a soldier on campaign would do. Without any sewing skills or experience, I used a big needle and some sort of heavy thread and stitched the tear closed. I still use the gum blanket either over me or under me at every reenactment I attend, and it now has a very authentic look to it.