I’m thinking of making myself a water bottle holder thing, because I like water bottles that come with holders that have shoulder straps, but I also have a water bottle that I really like that doesn’t fit in any of the holders I have. I know how to do a pointy-ended bag thing, but is it possible to do a flat-bottomed cylinder?
Sure there is! Here’s a couple. 
http://www.hjsstudio.com/bottle.html
http://flyingsticks.blogspot.com/2006/10/water-bottle-cover.html