-- Last Updated: Nov-06-12 7:47 PM EST --
....... you can use a dense foam board (like Dow Blueboard) and shape the mould to fit your deck area . Set the foam board piece over the deck , mark (trace) the deck's pattern on the foam board . Now you have a 1-1/2" or 2" thick (depending on which board you choose) pattern .
You can go several ways from there . You can shape the top into an arch w/ a sander . let the cloth and and resin overlap the sides if desired (this allows the deck cap to go over the gunnels) , you can leave a short ledge at the inside edge of the foam board form (to divert splash water to the sides) .... it's just a form you are designing , and then you lay up the cloth and resin over the form , trim the edges to desired length once cured , fare and round over , gelcoat or paint a finish on .
You are just making a shape that you like and using it as a form to layup cloth and resin over .
The options to make your mould are many , so use your imagination as you see it ... this post is just to give you an idea , where you go with it is up to you . The idea is the dense foam board mould .