i went very basic, both w/ the mounting technique and w/ the rigger models
the first season i had the boat i bolted 3/4" plywood to the corner of each rail and mounted the riggers right to that.
the next season i gave the plywood to dover marine and they gave me back aluminum plates matching the plywood size. simple and cheap.
BTW the downeast clamp-on rod holders are PERFECT for your pontoon rails.