That's how I build the base part for most of my little robots. Be aware that fiberglass can cause allergic reactions in your skin. Wear long sleeved shirts and gloves. You may also prototype with cardboard, wooden sticks (like those used in popsicles), duct tape, and a hot glue gun. If you want to mess with plastics, you can go to the junkyard and buy some inner fender covers from wrecked cars. Then just use a heat gun to mold or melt the plastic to whatever shape you need. If you make a mold with fiberglass, you can easily fill it with this plastic. It is quite durable and can be drilled/tapped. Bondo is another great thing to use, but be aware that it is very dusty and messy.