This is a case/enclosure for a fan speed control board. The one I bought is a generic 3-12V (2A) board but there are versions up to 36V as far as I know. The board design is often similar and simply sold by different companies/with different names. This is the board the enclosure is for:

To assemble the enclosure you actually need only the 3D printed parts. The board clips into the enclosure but it could become loose. Therefore I added holes onto the mounting pins to screw the board down with M2*4 screws. 
The cap is clamped into the enclosure and already pretty fixed but has holes for additional M2*8 screws too. With them it definitely cannot fall out and is safe to work with!
To mount the enclosure onto something I added two legs. These have 4mm wide (2mm radius) holes to use M4/M4.5/M5 screws.

Note: The opening on the right is used to remove the cap. Its quite hard to remove it because its clamped down really good.