You can get some 2" thick styrofoam insulation and cut rough shapes with a sabre saw. Stack the pieces up and glue together with hotmelt glue. Use a wood rasp and sandpaper to cut to final shape. Fill any pits or gaps with spackling compoung. Cover with fiberglass cloth and marine epoxy. (US Composites is a good source) Do not use auto body filler. (it will dissolve the foam.)You can mix collorant with the epoxy. When finished, you can dissolve the foam out with acetone. Here are a series of pictures of a boat I made this way. http://www.syotr.com/gallery2/main.php?g2_view=core:ShowItem&g2_itemId=15408