I've been working with 60mm between centres on the vertex joints, so you see that measurement employed again here. I positioned one of the stepper-motor mounting holes to the third corner of a equilateral triangle, and this set the motor spindle in a good position. The rest was just eye-pleasing geometry, no major science behind it. (These dimensions may not match Mendel printed parts exactly! They are my own interpretation.)
Aluminium is a nice material to work. I cut this with a hacksaw, cut the larger hole with a piercing saw, and rounded the external corners with a hand file.
Here's the bracket in place, with the stepper-motor mounted. It is very rigid once the nuts are tightened.
This view shows the purchased drive-pulley (mendel-parts). People fit that pulley the other way round also. I'm happy with it this way for now.