I modified the original Raspberry Pi Snug Case by Esquilo to accommodate the new 4 pin power over Ethernet header, and added snap in pegs where the original mounting holes were so the Pi can just snap in, no need for screws anymore. Since my Pi 3 B+ came with a heat sink for the Broadcom chip I also hollowed out a space under the case top to make it all fit together.
Printed in Solutech See Through Blue 1.75 mm PETG.
About the design process...
I run Ubuntu as my primary desktop, so I have been teaching myself FreeCAD since there is a native Linux version. In order to make the changes to the model, I had to import the original STL files and convert them to objects and then convert them to solids. After that I was able to modify the design and the re-export them back to STL files.