An alternative approach is to use the front brake as the rear with an external nut as FB recommended and adopt the rear brake for use in front as follows:

Drill the rear of the fork's crown out to 5/16 " (8 mm), which is easy, and get a long enough recessed nut to allow use of a rear brake directly. Those nuts are available as long as 35 mm which should cover almost any road fork's crown thickness. Here is a source for those nuts: Bike Tools Etc. - 1000's of bicycle tools and parts for the home mechanic!