So my teenage daughter is in theater camp this summer (proud mamma brag: she has the lead in "Meet Me in St Louis"). She was using a paper shopping bag from some mall store that she shops in until it got damp from her frozen waterbottle and fell apart. A couple of weeks ago, I whipped this up for her.
No pattern. Just winged it. (that's why one side is 1/2 inch shorter than the other. Oh well.
At least the side pocket fits around her water bottle.