That's a fair point. I can't think of any examples of round containers called cartons. On the other hand, an 'egg carton' has a specialized rather irregular

That's a fair point. I can't think of any examples of round containers called cartons. On the other hand, an 'egg carton' has a specialized rather irregular albeit broadly rectangular shape, so I don't think a carton is necessarily a rectangular prism. I think they tend to be that way since the material lends itself to it. Cardboard tubes are definitely a thing and I haven't heard them referred to as cartons though.