At first I carried my alcohol fuel in a smaller aluminum MSR type fuel bottle with a special cap made to be used as a fuel spigot. (from rei.com) When reaching in my bag i could feel the fuel bottle by the top of the cap. By comparison I also carried my field brandy in a 1L aluminum flask with a normal cap.

However i found I don't use crinkly plastic bottles in the field. So a 16oz water bottle will be used for nothing else but Denatured alcohol. Unless someone is reaching into my ruck for water and bypassing the drink tube or Nalgene bottle both on the outside and somehow missing the giant warning signs and 1 oz dash marks on the modified water bottle, no one will mistake it for potable water.