Re: What do I have?
My bow modification uses the existing structure, then I removed the plastic rims off the side storage and riveted L alum to tie in the side pieces on the top. I chose to keep a small entranceway, about 14", then I bridged an additional wall port to star to handle more decking and a seat base. Under that new section the area will house 24v of battery(2) for the trolling motor.
Your picture shows a full bow deck past the console. Wall it up and try to cut your plywood where it remains one-piece to the center bow deck cutout, and the two side pieces can be original.
Your other stick frame method versus a wall is a flooring, or STAGE method. Wall meaning plate and jack. Stage meaning ledger and joist.