You guys with stringer mounted seats- tell me. I have seen bent aluminum plate, or angles that are bolted to the stringers, and the seats mount to these. Example:
http://www2.***boat.com/image_center...t_Brackets.jpg
Does the seat flex, sway or tip very easy from side to side? Won't any movement crack the stringer?
That should never be a problem on a well built boat. But, if you have issues with that, buy C-Channel of the proper width and put the bottom flange against the floor.