Measure a spot in your freezer where your mold will fit, and cut a piece of foamcore to those dimensions. (My freezer has a drawer that's 15 x 11 inches.)
Cut four mold walls to match the sides of the bigger piece:
- 2 pieces @ 4 x 15 inches
- 2 pieces @ 4 x 11 inches
Mark each mold wall with a level line one inch from the edge. This will be your glue line-- the floor of the mold must be lifted from the table to leave space for the legs of the LEDs to poke through/splay out.
Glue up the walls of the mold box with a hot glue gun, applying extra glue at the corners and the "undercarriage" of the mold-- it must be a liquid-tight seal to prevent liquid silicone from seeping out!