One more case for Raspberry Pi Zero. This is simple, compact and quite rigid. 

The cover part has press fit with the main case. The board sits on four pins and from top is pressed by the cover part. Nominal clearances between the board and the case are 0.5 mm, USB/HDMI ports stick out of the board - this forces the assembly in a certain way.
To insert the board into case, first push down the GPIO side and then gently push side with USB/HDMI ports. While pushing USB/HDMI side of the board , you could try to slightly bend out this side of the case.

The board sits firmly in casing without any screws. Press fit ensures that cover part don't fall out itself, but you could disassembly it easy if needed.

Print settings:

• Layer height: 0.2 mm
• Perimeters: 2
• Infill: 20%
• Supports: No

Update

Added model variant (only STL files) with increased clearance around the board :

• Case.stl, CaseTop.stl - original model with 0.5 mm clearance to the board;
• Case_vB, CaseTop_vB.stl, CaseTop_vB_Closed.stl - model with 1.5 mm clearance to the board.

CaseTop_vB_Closed - alternative closed lid for Raspberry Pi without pins.