I have helped construct 2 onsite. They never went together well. I built one at home a long time ago that was a 4x4. It was solid and air tight, but OSB is not good for the sides.

What I would love to do is built one and frame it and use plywood on the outside and then take it someplace and have them spray foam insulate it. I think that would make it more rigid. And I could probably get away with 1/4 in plywood for the sides.

I got one out at my lease right now that one of the old guys built with pallets, and its heavy, not square and I am trying to seal it up better, and its not working well. I really think next year I need to just pull it apart and start over.