I started with a set of 4-pot Brembos I bought from a member of a motorcycle forum I frequent. I don't know what bike they're off of, but I think it was a Duc. I discarded the lines, MC, and lever, as they won't fit on my bike.
I first "mounted" one caliper to the rotor by attaching it to the MC and clamping the lever down. This located the caliper where I needed it for best fit. I then made a template from a piece of corrugated cardboard; actually, I cut up a shipping box.
I then traced the pattern on a plate of aircraft aluminum and cut them out on a band saw.
Once I had them to rough shape, I clamped one on and, with a punch, marked one mounting hole. I removed the bracket, drilled that hole, and then bolted it back in place. I then could safely mark the other mounting hole.
The plate was a little too thick.
So I sent them out to a buddy to mill them down a bit for a perfect fit.