The one I saw (Tea Chest Bass) must have been a plywood box with metal corners, and one side, the bottom, was open I assume to allow the thing to resonate better. I don't know if it was a real tea chest or just a plywood box. I thought it must be plywood because it was fairly large and took a beating and had no cracks, these guys whaled on their instruments! They are from Cardif and are called "Railroad Bill", very fun bunch. The string was attached to the center of the top of the box and went up to the top of the neck like on a washtub bass. There was another band at the same festival in italy that also had a tea chest bass, they were playing an interesting accordian/blues mish-mash, made me think they might be fairly common in Europe, but I really don't know.