go and get yourself a couple sheets of foam board. its cheap, like $6 a sheet, and you can cut it easily with a razor blade, knife, or jigsaw and slap it together with some tape to make templates for what you PLAN to build. you can see how it fits in the boat, if you need to trim it down, if you can sit in the drivers seat with enough room, does a cooler slide between the seats so you can put it under the deck, etc. then once you're ready break it down and trace out the template onto some plywood and really cut it out and assemble it...
some tips... for the seat bases, cut two. then trim a seat cushion base out of one of them. they'll fit back in snugly and your upholstery guy will thank you for leaving him something to staple into. dont forget to leave some room for foam. also seats that are too snug, rub. and rubbing vinyl on sunburns doesnt feel good.






