Being as I am in the fabrication industry and boats are my love I would love to be close enough to help you with this. I definately wouldnt use wood. Simply because of the added weight. If you have the necessary resources available aluminum is the way to go. I would suggest .063 aluminum for the seats. Using that the possibilities are endless. You would need to make a template that you could transfer over onto your aluminum plate of the bottom. Then it would be simple. Just figure out what height you would want the bench seats to reach and the width as well. Before breaking the "box" along the bottom where the bench or box sits make sure you add ....I would suggest at least 3/4" tabs placed every foot along the bottom edge that could be heli arc'd to the bottom. After forming the "shell you then could cut in any access ports for storage. Its not difficult if you have the necessary resources available.....the end result will last pretty well forever.... Depending on the use intended for the boat I would put a deck in the front with storage underneath.....a deck in the rear ...again with a rear hatch that would open for access to tank batteries pumps etc and in this rear deck is where I would put in a livewell that plumbs to the rear. The possibilities are endless.
Ayuh,.... Where's the Flotation gonna go,..??