I'd seen the idea here for one, some time ago.
I didn't make mine the same way though - instead just made the body of the holder out of one rectangle of fabric, sewed it into a tube, sewed across the bottom, and then sewed across two of the bottom corners to make a square bottom.
Not exactly sure about the fabric combination here, but Harry chose, and he's 4, so I suppose it could have been worse!
We also got a new vacuum cleaner today (after the old one blew up last week), and Harry spent quite some time this afternoon trying it out - with his trusty water bottle at his side of course!! He is very proud of it!