Made to fit for a DM860 stepper driver but others might work too
See drawing with measurements for detail.
.step available too if you want to modify it.
If you are looking for a DM556 holder, maybe have a try with this one:
https://www.printables.com/model/1307445-din-rail-stepper-driver-holder
The author remixed this model.
Scaled up to hold a dm860 and made the din rail mount "double"