I used these plans for an 8-pack carrier. The principles would apply but you'd have to adjust to the size of your growlers. I want to do something similar but don't have two matching growlers (yet). I am debating between the aesthetics of wood and perhaps a cloth or leather version (smaller, easier to store, etc.).
I think if I were to make this for growlers instead of the wooden dowel I'd use some kind of rope, otherwise the damn thing would be almost a foot and a half tall.
Share your plans and pictures when you're done. I'm curious which way you go.