the epoxy can delaminate from the clay. This has happened to me when drilling holes and test breaking bow/stern toggles. Once it has an edge, it just peels off. I had high hopes for Sculpey (and will post a few pics soon), but have resorted to more foam projects. At least so far, from what I've seen, epoxy gets a good grip on foam.

Consider carving it out of cheap builders foam or minicel foam until your receiver fits snug (remove) and apply a seal coat or two of epoxy, then apply small strips (band-aid sized) of fiberglass until it looks like you could drive a truck over it, then start playing with pigments until you find the color that you want and apply a few fill coats with that. That's the easy part. The door or front of the case would be more difficult, but the buttons...well, that's where I depart from this whole homemade GPS case idea, because I don't think I'd want to try that one. Garmin has buttons that would not function properly with a spring or extension... what about large holes drilled in the appropriate locations and insert soft plastic/rubber membranes. Now, that could work ???

Like the idea, Malcolm, but I think that true water-proofness would be hard to achieve by the homebuilder. I wish you luck, however, and would sure like to see what you come up with.
SMF