Construction for a homogeneous field using permanent magnets
The magnetic field in the region between the arms would be relatively uniform in direction, but non-uniform in intensity. Below, I've attached an image of the field that I made using very simple simulation software (note: this is extremely simple software meant for didactic use only. its results are not necessarily representative of reality); notice how the magnetic field is more intense (whiter) on the left side, near the permanent magnet, and less intense (greener) as you move to the right.

If I adjust the intensity a bit to better show it, you can also see that the field between the arms is far lower in magnitude than the field nearer the permanent magnet:

This may be good enough for you. Or it may not. Personally, I'd use a Helmholtz coil and just deal with having to wind the thing manually; it's tedious, but at least you only have to do it once.