Well, here's what you could do instead. Make the part out of foam. Make it to the exact dimensions that you want the final part to be... THEN! instead of trying to reinforce the foam, build a box around it and fill it with RTV silicone! Then you can fill it with your favorite urethane and paint it, machine it, make a billion of em for that matter. If you want, check out www.smooth-on.com They have all kinds of wonderful castable plastics.
BuT, if you don't want to bother with all that, then yes, you could just offset your boundaries and laminate glass over it. I'd advise looking into vacuum bagging too because chopped mat and wet-layups are just asking for rough finishes and air bubbles, especially in small parts.
I could go on for hours with any of those reproduction methods, or some others i didn't mention, but let me know what you're interested in and we'll talk.