The one good thing about the ABS plastics is that they make excellent molds. All you need to do is get small quantities of mold release wax and PVA. Wax the inside of the plastic parts, apply a coat of PVA with a foam brush and when that dries apply the cloth. I would go with a surface layer of 1.4 oz cloth followed by a couple layers of 6 oz. The 1.4 keeps pin holes from forming. After a 24 hour cure, pop your new fiberglass part out. The cowl and wheel pants on my Laser were made using this system.