[TunaMeltdown]

You could try a different rod holder to help with the flexing issues. The Lees are great but they only have 3 bolt pattern and less surface area on the top flange. Your flexing issue is coming from the 3 bolt pattern. You shouldn't have to box in your gunwales and coaming pads. In my opinion the bolt holes are to far forward on the lees. When you mounted it I bet you saw the back of the rod holder lifting up? Same thing happened to me and I switched to LD Marine rod holders. They have 4 bolt pattern and a larger top flange which distributes the pressure put on the rod holder buy the rod. This is the same rod holder used by all the giant bluefin tuna fisherman. I've fought bluefin tuna up to 80" so far with mine and there hasn't been any flexing issues.
http://www.ldmarineproducts.com/