I found plenty of mounts for Arduinos and PIs, but nothing for the STM32 Nucleo. So I made this. My board is the STM32-F446RE. It mounts to the 3 standoffs with M2 20mm bolts (18mm will probably do). 

There is space for a small breadboard, the common size small board will press fit nicely, but you can also use the sticker on the bottom of the board to affix it in place.

Two mounting holes accept M5 bolts for affixing to a base such as 2020 extrusion.