I think a box bike would be great for dogs, but would be a very expensive option and not great for long rides.
My little dog (20lbs) rides in a burley tail wagon trailer and loves it. My Golden has trouble fitting in the trailer since he is taller. If I was going to pull him often I think I would modify a trailer like in this article, to fit his crate.
https://www.xtracycle.com/blog/hound-haulin-on-a-long-tail-cargo-bike/