Hello! you can download the design files for the fretting mechanism and the pure data patch for using it with an Arturia beatstep here: https://github.com/JacobTFH/adapted-bass
I used 5mm plywood for the fretting mechanism, and I also made additional spacers to attach to the arms out of 5mm ply.
You will also need some 6mm threaded rod, 6mm bolts, zip ties and springs. I used these solenoids: https://uk.farnell.com/ped/42-120-611-620/solenoid-12vdc-with-spring/dp/1207156
Bear in mind there was a lot of trial and error involved in terms of attaching the springs, spacing the fretting arms, adjusting the solenoid etc. You'll probably need to adjust the design files to fit whatever size/shape fretboard you're using.