I had a small Arduino Nano project where I just needed to connect a few wires directly to the pins, in a very tight place. I couldn't figure out any other way of fastening the board than designing this holder. Works well at least for my purposes.
Turn it on it's "back" when printing - the result will be better that way.
Hint: the holes are intended for making hot glue to stick better. Of course you can also use screws, if you find a suitable size. If you use screws, you might want to tinker the model and change the holes to your liking.
This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/jb7f7A7plI6