How do you optimally set a patient up for breast radiation therapy if you don't have access to a breast board or wing board?

"Optimally" is a continuum and hard to define.

Without a breast board or wing board, a vac-lok bag might be your best bet. You can often move much of the "beans in the bag" superiorly in the bag, so that you can create something to support the patient's ipsilateral arm(s). Plus, moving the beans superiorly might give you enough mass so that you are elevating the shoulders off of the treatment table (sort of emulating what the breast board does). You could even put one of the large Styrofoam triangles under the bean bag to get you some elevation (again, emulating the breast board).

To answer more broadly, there are several distinct goals that are worth separating.

The angulation (i.e., elevation of the shoulders) is most helpful in patients...

I like the vacloks as well. By being creative with moving the beans around, you can build up a great arm cradle and get a very reproducible setup, especially in whole breast treatment where margins are measured in centimeters and not millimeters.