Well, Hubs has figured out what this whole sewing thing can mean for him. I don't like the elastic in my workout pants...no problem, my wife can sew! I want my too short pants turned into shorts...no problem, my wife can sew! There aren't any drink pockets on my favorite backpacks...you get the picture.
I've done all of the figuring for a Camelback water bottle (picture below), so if you happen to have one and you happen to have a backpack in need of another pocket, you're in luck! You'll need some backpack-ish fabric (1/4 yard is plenty), elastic...and I guess that's about it other than thread and my super-helpful instructions (that's the blue sticky note).
First, fold over the top of the rectangle to allow a gap for the elastic. Pin it down and sew across.