I decided to go for wide mouth canteens and I am still happy with the decision. They are much easier to fill and to clean. The option to easily add ice cubes is nice to have but no real advantage to me (YMMV).
The are plenty products that allow to drink as easily as from a narrow mouth canteen. I use the sipper insert:


I am satisfied with it and you only need one. It is easy to put in or take out and it does not add bulk.
There are no converters that add the advantages of a wide mouth canteen to a narrow mouth canteen.