first use ms paint and give us a head up view of what you want, we might under stand better. as for long sheets i would make it with a lapped peice of wood under neither the joint, you could bring it in, in pieces and glue it up in the boat. ----===---- some thing like that for the lap joint. also if you seal up the space under under the settees you might want to ad a drain plug so if/when you get water under there you can pull the plug and let it drain to the bilge.
as for mocking things up to see how they fit, i would use blue insulation foam, they sell it in 3/4 inch thick sheets, easy to cut and move around. when done mocking and building you can use the foam for insulation