I researched several board mounts, but wasn't completely satisfied with them. So I designed my own with Fusion 360.
It expects a self-stick 830 point breadboard.
I have used this with Uno, Mega, Elegoo R3 (Uno clone), and Adafruit Metro M4.
Nothing special for printing. Simple default settings are OK: no support, 20% infill, 0.20mm, 2 perimeters, PLA.
The author marked this model as their own original creation.