a container/box to put stuff safely away or out of sight.
It's a tight fit. If you think it's going to lock. Try rubbing it in with a drop of olive oil to reduce the friction of the material.
It's Like 2 people who are both perfect as individuals. But still need to smooth some of the edges to really make it work.
round version can be found; http://www.thingiverse.com/thing:582468
(or if you can controle the flowrate for the outer layes, drop it for I think about 10-15 %)