Vacuum forming mold masters for making ice luminaries

These are prototypes for ice luminary (aka ice lantern) mold masters that I'm experimenting with for a friend at our local Makerspace.
As I understand it, the mold masters have plastic drawn down over them by a vacuum forming machine. The vacuum formed plastic parts are then filled with water and frozen. The ice is then removed from the mold and an LED "throwie" is placed in the pocket under each one.

These have to be kept fairly simple so that the mold master and the ice can be easily r