Just went and Checked out my copy of SoD and made an interesting discovery.

All boxes the Mitd has ever been in have had the corner braces, so they have been reinforced.

The box he is in at the circus is identical to the one the hunters used to capture him.

BUT the box he is seen in outside the Dungeon of Dorukan when he is carried by the zombies is different. Before that point the box is always shown with a door in the front, but that last box and all boxes hehas been in since have no such aspect.

This kinda invalidates or assumptions about the weight of the box in the Circus scene (using the zombie carrying capacity) since it appears to have been a different box.

So back to estimating the weight based on materials. The british so oak thing could work, otherwise we can assume they built the box on sight and thus made it out of wood from the jungle where they captured Mitd.