Mr. Army
Not to step on your idea, but have you thought of eliminating the hinge and just going with a traditional key dupping frame?
I have taken the liberty of modifying the 'Clam' design to a pegged design.
This one uses 'knife edge' sides to eliminate the bleed holes, though they could be added.
The little clay pool at the rear is used to get a profile of the key for blank matching.
The entire piece could be produced on a mill using only a 0.75" ball end milling cutter and a 0.1875" plus an '11' or '12' wire number size drill bit. With a CNC machine this is cake.
The rear pin is tallest, with the two front pins one a bit shorter than the other. To close/register the shell you slide the rear pin in it's corresponding hole then swivel till the next tallest drops in and the last one will align. The whole thing (as well as your design) could be closed with a 'giant' binder clip, the largest I've seen has a capability of 1.25".
I know nothing about FIMO clay, but I would be sure it didn't adhere to the frame on heating, it would suck bad to bake your frame and have to hammer the hell out of it to remove the clay.
If you wanted a bakable shell I would use aluminum, but if you were to settle for using just modeling clay you could make them from UHMWPE, it's tough stuff, though it can't be heated, and it machines fine. It's what the 'Clam' is made of.