See photo below for my measurements. Fellow P4'ers, please help to confirm.
The idea is to machine a horseshoe that bolts into the 2 brake bosses (the top 2 holes in the photo). Then, have a 3rd bolt going into the bottom of the photo hole, to add stability. Then, somewhere very close to that bottom hole of the frame, have a hole drilled out on the horseshoe to run a standard, single bolt rear brake (best options are TriRig or Magura).
Thoughts? Please share more details and opinions so we can hopefully have someone build this.