Well, I'm kinda new to this but here's my solution.
I have already put a shelf in mine for 'Buster' to have more room in so I put 2 screws about 2 inches apart with the screws not all the way in, through the bin and into the shelf (drilling holes first ofc).
I had some mains cable laying around so stripped out the cores, but i guess you can use any kind of stiff wire as its on the
outside of the tub, wrapped one end of each piece around the screw heads. I made an oval hole for the spout to fit through (making sure the height was ok). I did just a round hole to begin with and wondered why it was sitting at a funny angle! *Doh*
Then just twisted the cables together to hold the bottle in place.
Pretty easy realy, 2 screws, 2 bits of wire and a dremmel. Oh, i put the bottle on the outside so that there was more room left inside the 'bin' for Buster to go mad in
Hope this helps..