ATTENTION: THIS IS DESIGNED TO BE MILLED FROM METAL!!! This design can not be 3D printed out of plastic.
This is my take on a passively cooled raspberry pi3B+ case.
The top case makes direct contact with the CPU and USB Controller. I milled mine out of 5083 Aluminium.
Only the external facing ports of the PI are accessible with this design, it does not offer access to the GPIO header, the DSI and the CSI Port.
M2.5 Screws are used to hold everything together. Place some rubber O-Rings between the lower part of the case and the Taspi to gently push the board against the top part. Place some thermal paste on the chips on the Raspi for optimal heat transfer.
As heat is not really an issue with the pi3 the top case is available in two version. One simple, flat Version, without any cooling ribs and a second version with cooling ribs for better heat transfer to the air. Both versions work fine on my models, I run my OSMC instance in a case without cooling ribs and never experienced heat problems.
The bottom part is available in two versions, one with mounting options and one simple version that does not offer any mounting abilities. Choose the one that fits your need.