If you're happy with a 45 degree angle on the arm and the amount of differential that provides then you can just put a + arm on but angled so it's a X shape.

Remember too that when you're working with arcs like this that you are also introducing a non linearity in the amount of throw compared to the stick movement. Not so bad on the one that is pulling in Pauls second pic but on the other one that is pushing the arc travel is reducing quickly as more throw is used. So the differential %'s will be very low for smaller travel amounts and higher for extreme stick positions. But that's OK since the adverse yaw you're compensating for is more noticable with full throw than at small movements.