What I did on my old boat was to mount the seat onto marine plywood the length of my dry box, then used the straps that hold my dry box down over the plywood to hold the board in place. I also mounted small ammo cans on the sides for quick access.
When I needed to get into dry box, the dry box lid would lift the seat board up. When I needed to load the dry box, I'd take the board with the seat to the side. Worked pretty well and was cheap to make.
I don't have a decent detail picture, this is the best I can find: