Mounting Pedestal Seat

[Greyduk]

I have a 1987 V-20 cuddy. I am thinking that I would like to mount a helm seat opposite the one that is already there for the driver. The problem is, I want to be able to put a backing plate under the deck and bolt thru. I would either have to put a deck plate beside the pedestal in the floor (which I don't want to do), or go in through the side of the fish box which is in the floor.

Has anyone put a deck plate or cut a hole in the side of the fish box? If so is there anything that would prevent access to the through bolts for the seat if I go at it that way?

I would appreciate any thoughts on solving this. Thanks for any input.