I don't worry so much about making the joints with finger joints, but instead use rabbets. The thinner area, where the rabbet has been cut to accommodate the frames, is certainly the weakest part of any super. Personally I have adjusted my super making plans accordingly. I do not cut any handholds in the sides of otherwise, perfectly good, super wall boards, and I attach cleats to the top ends of all supers, this reinforces the weakest part of any super (the frame rest rabbet), and provides excellent grips for handling supers. This only requires covers that are compatible with the extra length the cleats add to the top surface of each super.