This one goes a long way back but it works. As in most prodedures it requires a form made of whatever is handy, balsa, hardwood or whatever will hold shape. Cut some of the old fashioned paper grocery bags into strips and soak well in thinned elmer's white glue. apply to the form in several layers. Make it thick enough to sand to shape when dry. Works well for compound curves but cannot be relied on for strength. A layer of fibreglass on top will help make it stronger. Cheap and easy, which is a major requirement for me.