Epoxy is a two-part liquid that eventually hardens into a solid material. To create a mold from epoxy, you need need a frame to hold the liquid epoxy until it hardens. The mold will consists of two halves, so the frame must also be constructed as two halves that fit together.
This is a pre-made aluminum mold frame that I purchased from LNS Technologies at - https://www.techkits.com/
I could have tried to make my own frame, but this one was affordable and already had steel alignment pins to align the two halves.