In Canada Stainless Steel Water Bottles are readily available at Canadian Tire for around $12. They are a fantastic improvement over trying to improvise a plastic bottle and are more robust to damage as well. If you carry a Water Bottle, it Should be Stainless Steel!

I buy a bottles with the biggest mouth and then cut off the plastic attachment for the lid. I then re-configure the mouth of the bottle with wire and two o-rings to allow easy use above a fire. The lid is then attached with cord so it can be easily detached.