I have found that using alphabet letter moulds for candy work well. Make a reverse image by using the reverse side (convex)of the mould. I have found that a product called "Rock Hard Putty" works well for making the new mould and also casting of the individual letters. I have used everything from "play Dough", silicone caulk, Wax, and homemade flour/water dough. The "rock hard seems to work best for me and if baked in over at 300F for 30 min they will take a lot of use (also bake harden the mould)

use non stick cooking spray, wd40 or even Pledge for a mould release. Spray all moulds (after they come out of oven) with a clear acrylic spray, several coats allow to dry between coats. this process works well. I have even cast some reusable letters using polyester auto body putty (Bondo)& JB epoxy weld and the mould will even stand up to casting melted "pot Metal" letters Do not use the acrylic spray if casting "hot Pot metal". It works well for me as I don't want to pay out 100's $$ for sets of cast reverse letters to experiment. E-Bay has a huge selection of plastic alphabet candy moulds for 2-3 $ each