you could use the bottom of the seat you removed, then pull the upholstery off and then use the foam and steel rod as a 'mold' and fibreglass that thing! you have options to deal with the top...you can either use more foam to lay across the top and make it solid or you can choose to use some wood and piano hinges to make it a 'flip-top' kinda like crx's. you will have to trim the remainder of the foam that you don't use as mold, and you may fibreglass the inside of now existant frame. you'll want to upholster the fibreglass and whatever top you decide on doing.