Variable voltage mods are ones that have adjustable voltage like the Buzz and Little Sister, not ones that accept different battery combinations. You might want to reask your question and not specify variable voltage. Based on the title you’ll only get folks ready to recommend a true variable voltage model. It would also improve the recommendations if you included more info on what you’re using now, your preferences and a more exact price range.
I’ll give another endorsement for the Buzz by the way. I’m very happy with mine.