I had my seat made for me he used 3/4 Plywood and coated it with fiberglass. As the previous post said you need to support the center. It was a problem for a lot of 205 owners. On another note I have left my back seat in the garage for the lasty ear. It gives you alot more room behind the engine. WE boat behind the house so seating in not an issue.
Here are some pics in an older thread