All it takes to make a heat retentive plate is:
1) Modify the mold to add a separator and centering boss on the underside of the plate
2) Assemble the heater upsidedown using epoxy resin
3) Assemble the cover of the heater or base also with epoxy.
(I have been using AREMCO 315 (TM) for prototypes, but there are many other epoxy resins available, Check BONDMASTER (TM) for instance).
HotSmart technology, Zap and serve (TM), InstaHot (TM)