Setting the field at all boundaries works for creating a uniform field region once you realize that the "Magnetic Field" boundary condition specifies the "tangential" component of the field in absolute coordinates -- a somewhat non-intuitive convention (at least to my mind).
So, for example, if you want 1 A/m field in the z direction, 4 sides of your boundary cube will specify 1 A/m z component, wheras two of you cube sides will specify zero z component!!!!!
So, for example, if you want 1 A/m field in the z direction, 4 sides of your boundary cube will specify 1 A/m z component, wheras two of you cube sides will specify zero z component!!!!!