OK, I understand now. I'm a mech engineer so I can appreciate design compromises. In fact, a good way of looking at design challenges is to view them as conflicts. The resolution of design conflicts is what produces a good design.

So, I was reading a book on the history of Boeing. Apparently, they had a lot of trouble in the early days (pre B17) with inadequate yaw stability. The results are very evident with the B17 and B29 designs which had gigantic vertical fins. Anyway, one thing Boeing used on several aircraft was triple vert fins. This would be very easy to implement in your design. They wouldn't need rudders either. Another thing you might try is using a symmetrical airfoil shape. This should help make the fin more effective at a given angle of attack (yaw angle).

Another thought I had was regarding aileron differential. If you have a very heavily loaded wing operating at high Cl, you probably have adverse yaw condition when using ailerons. Aileron differential can counteract this. A good place to start would be 50% more up aileron as down (3:2 up/down ratio). Some model gliders use differential ratios as high as 4:1. The idea is that the drag due to deflecting the ailerons is equal on both sides of the wing. In some cases, like the glider example, a little induced yaw in the turn direction is desirable. Adverse yaw, yaw opposite of the turn, is never desirable.

Good luck.