[Bicycle Bob]

Basically, you are developing a buck to pull a fiberglass mold from. You can make it from any kind of garbage that can be polished and is strong enough to support wax, wet cloth, and a paint brush.
If you are planning on a single skin construction, and don't want any of the usual grooves, steps, or ridges to add stiffness, you can make a separate molding to produce an internal support, just like you find on a hood or trunk lid.
The "right" material is the one you most enjoy working with, provided it can make a suitable shape. I've done a lot of detail work using wall patching compound, and then hardened its surface for polishing with Polyvinyl Alcohol, a release agent no gentleman's molding shop should be without. Shellac also works. Final polishing can be done on the mold surface itself.
Material choice is important. Ten percent of the homebuilt aircraft that are begun are finished by the first builder, ten percent by the second builder, and ten more by the third builder, who gives up on 70% of them. You have to enjoy the work, not just the vision.