This phone case was entirely designed by myself completely from scratch only using Sketchup and a caliper. 
The two main shells need to be bolted together with M2 Screws. The case was designed to be printed from two different colours. I used grey ASA filament in combination with neon orange ASA. Use acetone or glue to weld the components together.

Additional parts needed for assembly:

-18x M2x5mm Screws (countersunk head)

-9x M2x10mm Thread spacer (commonly used as spacer for circuit boards)

-A little bit of foam/sealant-tape to add some thickness to the buttons and avoid the phone from moving inside the case

Instructions:

Print all the parts from whatever colour/material you prefer and combine them using glue or acetone in case you use ABS or ASA. 
Make sure all the holes for the screws are countersunk. They may be clogged with support material. Use a 3mm drillbit by hand to clean them up if neccessary. Apply some sponge/foam-tape or whatever onto the inside of the backside shell as well as on the buttons inside surface. 
The case is designed a little bit oversized to prevent tolerance issues. Depending on your printer you need to use more or less tape. The buttons usually only need a half milimeter or so. Use a sharp blade to cut the tape very thin. 
Apply bolts and tread-spacers onto the backside shell already and tighten them. 
Put the phone in, add the buttons and screw the front shell on top. Be gentle with the torque on the front screws. The cutouts for the threaded spacers are oversized to allow the front and the back shell of the case to get pulled against each other to avoid any gaps between them.