Howdy
On the 21ec the rod holder are 30 degrees, I think. Scotty makes a rigger mount for 30 degree holders. However, I don't think I would do it without some sort of backing/reinforcement plating, and I'm just not sure how you would do that with those types of rod holders. I had a scotty pro pack electric rigger and the are heavy and will put a significant strain on any mount. If it were me, I would mount a horizontal mount with a strong backing plate underneath. On the 21ec, there is room for about a 4" square plate mounted horizontally, just forward of the rod holders. Big Jon riggers might be a better choice because the have a smaller mounting footprint.
Cheers,
Gregg