If the shape is similar to an existing one, locate a used mold and some dremel tool time is fairly easy, modify the mold. Simple molds can be made from pliers and small blocks welded to them, but of course dremel tooling steel is more time consuming. Most silicones will not hold up to repeated casting, some epoxies, alumalite, etc do, but are pricey. I quess it depends on your design, and equipment access. Aluminum bar stock from any mailorder supplier is inexpensive, easy to machine, will last forever and one length of material will keep the creative juices flowing for a very long time. No need for hinges, just a couple of alignment pins, clamps to hold the halves together. Important thing is, don't give up on the idea, where there is a will, there is a way!
