Haha, I've built a lot of stuff out of 80/20, and I don't think I've ever managed to get all my slide-in connectors inserted without having to disassemble part of my project after forgetting them! The roll-in connectors solve the problem most of the time but some of the special connectors have to slide in.
FWIW, if you put the one cutout on either the left or right side, you'd have access to the front slot from one side of the rig, and access to the back from the other, although perhaps the desire for symmetry outweighs having access to two slots.
This is a great design BTW, I've been window shopping rack solutions for a while and this is looking like it might be the best fit for my needs.