Hi there @outpac94!
While many purifiers are designed to fit with wide mouth bottles, there are some that can be inserted in a narrow bottle far enough to create a seal between the "pen" and the bottle so that the bottle of water can be flipped over and the water treated.
The Katadyn Ultra Water Purifier is an example of a purifier that should work for you in this case.
Hope this helps! Additional information on water treatment can be found in our Expert Advice article about choosing a water filter or purifier!
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