As my daughter began to meaningfully use her arms with the arm floats, another issue arose. The flats just didn't lift her arms high enough. She could lift her arms 6-8 inches, but the arm floats stopped providing additional support at that point-- and they even prevented her from lifting her arms higher. The obvious solution was just to make the z-rods longer, which would provide for more z axis travel (i.e. arms can go higher).

The old design is shown (bottom) along with the new design (top). It's a relatively simple change to offset the hinge points for the two rods. There's a trade-off being made here. While the highest vertical displacement is increased here significantly (which is what we want), we trade that for a restriction in lower vertical displacement. You just can't move your arm down as much after this change. But, as it turns out, that's no big deal. In actual use, we never came close to the lower limit, but we were constantly running into the upper limit. So it's a good trade-off to make.