This holds an Adafruit 1/2 sized (or full sized if you don't mind it sticking out) Perma-Proto Breadboard beside an Arduino Uno, Leonardo, or similarly sized board. The design is slightly tweaked from the photos in that the hole through the nub near the power plug was removed. It works great with two 3mm screws even without bolts when 3D printed thanks to the natural "threading" of the plastic.
This thing was made with Tinkercad. Edit it online https://tinkercad.com/things/akTdswmJMVd