The fleece idea is a good one and works very well. You can either make your base from a wood structure or foam. If you make it out of wood the wood structure must be covered with something like plastic packaging tape to keep the resin from bonding to the underlying structure. If you use foam, it must be a foam that polyester resin will not attack. Dow makes a blue foam that is primarily used to insulate pipes in refineries, it goes by the name of Trymer. It sands extremely well and can be shaped into any form. It allows you to make flowing lines and such. Once you have your shape formed then you cut a piece of glass cloth roughly the size of the shape you are going to cover. Spray both the foam form and one side of the cloth with a good grade of contact adhesive. Do not use 3M #77 as the resin will interact with that adhesive. Then the cloth can be placed on the form and it will adhere to the curves and smooth out very well. The contact adhesive will not affect the resin at all. Once it has cured you can either leave the foam form or dig it out. You may have to apply more than one layer of cloth depending on what weight cloth you use. Smooth it out with body filler and paint as desired.