advide on making weedeater boat?

Hey i am wanting to make a rc weedeater powered double hull style rc boat about .75 meters long , a guy at my school has made one for a tech project but has left and i wanted to make the same thing, what i need to know is how should i shape the fibreglass into a hull and get it even, i was thinking of doing it with atype of polystyrene because after i have finished i can just dissolve the polystyrene and have a hollow hull but the foam is too coarse to mould into a even shape and i was wondering if anyone had come across a higher density foam or another method. thanks
and also if anyone knows of any websites with instructions on this could you please list them .

thanks

I started on making one i got the fiberglass ready but i dont have the rc techno. first you make the skeloton from cardboard.