How do the cup holder mounts fix into the cup holder and would it be strong enough to hold a lowrance hdi5

I think if this method is the best ill knock a bracket up at work so any improvements i could make.

Any recomendations on a plotter sounder for this ski other than the lowrance. I will also want to whip it off to use in the tinny.

Also right now ive got a cheep esky with rod holders bolted on etc, are those purpose made ones worth the dollar?