A while ago, I was asked to design a Raspberry Pi 3B mounting bracket for the Ballarat Hackerpsace when they were running one of their excellent Raspberry Pi 101 workshops.
The Hackerspace wanted to be able to mount a Raspberry Pi 3B on the back of a monitor that had screw holes to suit a Vesa 100 (Vesa Variant D) monitor bracket.
So, with the help of Firnsy from the hackerspace, I set out to design a mount for the Pi's, and this is the result.
If you find this design useful then please like it on Thingiverse  or MyMiniFactory

Parts Required:
The Mount - 3D Printed / CNC Routed / Laser Cut.
M3 x 18mm screws x 4 - hobby store / eBay.
M4 x 25mm screws x 4 - hobby store / eBay.
M3 x 7mm spacers x 4 (I used 7mm spacers because they were what I had, but you can use any size, you just need to use different size M3 screws.) - hobby store / eBay / 3D Printed.
M4 x 10mm spacers x 4 (Same as the M3 spacers) - hobby store / eBay / 3D Printed.
M3 nuts x 4 (If you adjust the hole size then you can screw straight into the plastic and not use nuts.) hobby store / eBay.

This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. By downloading these files you are agreeing to abide by the terms of this license.

Download STL

Download Source (F3D)