One product you may wish to look at is RTV potting compound from "Alumilite" (http://www.alumilite.com/Category.cfm?Category=Silicone
Rubber) Their "Quick-Set" seems to work well. It may be wise to spray coat your project with a mold release agent so if you were to have to open it up later it will be easier. You may also wish to consider adding your mounting screws before casting your mold as almost nothing will stick to the RTV. The "box" for the mold can be made from Lego blocks, foam core, a plastic box, etc. Hope this is helpful.