Kajun,
I had a jon boat almost exactly like yours a few years ago. I know you said you don't want to cut the bench seats but thats exactly what I did. I carefully cut out a section and saved the piece of aluminum, then mounted my seats with some 3/4inch treated plywood underneath. I took the saved square of aluminum, some pop rivots and a couple small hinges and now had a storage area. It worked for me.
Later,
Buzz