Arduino friction mounting plate with small breadboard addon
I would recommend printing this at around 2/3 complete very slowly and with max cooling, the pins are very small and warp easily, the overhang is manageable with max cooling and slow print speeds. The .gcode file included has code to support this automatically.
The Arduino fits in and is held with friction, it may need more for a vibration heavy environment, also included is a simple hook for temporarily supporting the mount (in my case on a radiator).