Heres what i came up with,finally managed to upload it,basicaly its one piece of insulation foam board cut to shape then epoxy coated,thin cloth might make it stronger but im not sure its needed to be honest.You could use blue foam also as its a little better quality but white foam is cheap which i like
.Another idea i had to strengthen the foam board was to use a piece of coraplast glued onto the top,i made an airboat like this but i sheeted the whole thing which is also another idea,seal all joints with silicone sealer and its water tight.
Mart