My sideboards on my 146DD frame are a foot wide and have a 3-4" gap between the dry boxes. I kind of like this since it has plenty of room to drop the boxes in but I also regularly stick a water bottle, throw bag, shoes, or whatever other loose item might be floating around on top of the boxes that I don't mind getting wet. Its obviously not a "Rig to flip" situation with that stuff unless you clip it in, but I definitely utilize that space to secure stuff.
I think a foot wide is plenty too... I don't feel like its prohibitive to the purpose to have them that narrow. I can walk around the boat no problem and it provides a nice stable platform to step on and off the boat.