Hi Deflation,
Basically if you are going to grow a tree in a container with the aim of thickening the trunk, you want the biggest container you can get (or at least one that's its going to take a few years for the roots to fill). So long as its got good drainage, it doesn't matter what it looks like.
Vance Wood makes some very nice training pots that look very nice, are well constructed and have horticultural benefits to the trees as they have mesh sides - thereby allowing for increased aeration of the roots.
They are also very reasonably priced. Unfortunately I live in the UK, but if I was the other side of the pond, I'd place an order on him for a few. Try sending Vance a private message, as I'm sure he'd be happy to give you a quote on them.
Also read this thread -
Training Pots 101 as it contains some good information on exactly this subject.
All the best,
Aaron