I could not find any designs for boxed made of 100% TPU, so I came up with this design. It uses a TPU band to hold the two halves together and has no moving parts. I printed this using TPU 95A (overture speed TPU) with a layer heights of 0.2mm, a wall thickness of 0.7mm, wall count of 6, top and bottom shell thickness of 8, and cubic infill of 50%. There shouldn't be much if any infill actually used with these settings though. 

This model was designed using onshape and can be freely copied and modified using the below link:

https://cad.onshape.com/documents/4df74353a3fa1fe72415292b/w/ea23c51c2dd7e77d31b18e3d/e/e0a4dc9b956a378bea6fe3b2