This is a simple modular project box I designed to house small electronic projects. The assembled project box is roughly 75mm by 40mm by 60mm but can be scaled up or down, within reasonable limits*. 

It has slots inside so that a PCB can be slid inside. I recommend printing the end caps and faceplate entirely out of bottom and/or top print layers so that they can be easily drilled to create various openings. I added a slight bevel to the edge of all parts to alleviate elephant's foot issues and facilitate joining the parts together**. With the clearance I chose for my specific printer, the parts fit snugly together so that no glue or fastenings is needed for the end caps. Your results may vary though depending on your printer's resolution.

*Note: if you scale the box up or down uniformly, the PCB slots inside may no longer work for their intended purpose, but will still help with the rigidity of the box. In addition, the sides will get thicker or thinner accordingly, as will the clearance between parts. (Scaling non-uniformly will help maintaining clearances though, if you know what you're doing…). This is why I say: scalable within reasonable limits… If there is an interest for other sizes, let me know in the comments!

**Note: The prototype box shown in the photos doesn't have the beveled edges...