Amazon sells strips of industrial strenth velcro
2" x 4" strips - less than £3. Here's the list of sellers as not sure which post to Ireland (below).
You just cut to size, stick one part on the tank, the other part on the bottle and press the two parts together. It's supposed to hold really well. Probably best to just have a piece big enough so it doesn't show so not visible for chewing.
Amazon.co.uk: Buying Choices: Velcro Heavy-Duty Stick On Strips (2) 50x100mm Black VEL60239