Each child will be able to pick their own materials next week when we get to see them. Adjust measurements if want to use it for larger bottles. Would work for re-usable bottles, too.
After sewing that and turning it inside out, I butted one end up to bottom of main part and zigzagged them together about 1 inch from side, then sewed other end of bottom, too...had to scrunch it up a bit but it worked out ok. Sorry picture showing it under foot is so fuzzy.