I used super-sculpey (or sculpey-firm. basically the same) to sculpt the piece.
Then I sprayed it down with a good coat of Crystal Clear. Since then, I do not use Crystal Clear anymore. It works, sure, but it stays tacky for a long time. Now I use Mod Podge Acrylic Sealer, and I absolutely love it!
Anyway... then I use a Van Aken Clay (or the substitute) to press the piece into and make some keys.
Then some legos around that to build walls. **Don't use Black Legos or rip-off legos, the finish dissolves and sticks to clay**
Then spray some universal sealer, mix my resin, pour it in each side, wait until a speck of white appears, then quickly and carefully put them together, slap a couple index cards on top and bottom to help keep it from "squishing", and snap some rubber-bands around it.
10-minute count-down, and... bam! Chest piece. Trim, sand, smile, on to the next piece. :)