@chuck123wapati, I haven't made a canteen, yet, but have made several mugs with just beeswax and what I do is I put the mug in a food saver bag with beeswax and then seal it. In a pot that can accommodate it, I use a silicone oven mitt and start submerging the mug in the very hot but not boiling water until all the beeswax has liquefied and has coated the mug. Once you are sure all the leather is impregnated with beeswax, take it out of the pot and out of the bag. Ensure the shape is what you want it to be and then let it cool. Keep the beeswax in the bag and submerge it to keep it liquid and once the mug is cool enough, pour the beeswax back into the mug and slowly tilt it to cover the inside of the mug with the desired thickness of wax and pour out the rest. After the first one you figure out a lot on how to do it better on subsequent ones.
I say all that to say when I get to making the canteen, I'll approach it similarly. Not sure how to keep the belly on the canteen though, maybe just pour in the liquid wax from the start? YouTube probably has some answers too.