A box with lid.
On one side, the lid is connected with the box by a hinge, on the other it hooks to the box upon closing. Both the lid hinge and the hook are connected with spring-like elements to the lid, which forces the lid to adhere to the box. However, it also makes the lid bend itself, so that a slit forms between the two other sides and the lid.
The lid is formed so that the boxes stack on each other.

Obvious manual assembly required. The design assumes that a piece of 1.75mm filament is used for the hinge rod. The holes for the hinge rod may need drilling if extra material ends up there while printing.

This model has been developed up to a functional stage, but once I've had it in my hands I realised that this is not the box I'm looking for. May not be as comfortable to handle or durable as intended, for I dumped it in favour of another design and did not do any finishing touches on this one.