This process should work for many different types of items or models. Note that the original item must have a parting line without undercuts. That means the model must be able to be separated into two halves from which the new molded part can be easily removed.
Here are the steps:
Step 1: Frame for holding the epoxy.
This is a pre-made aluminum mold frame that I purchased from LNS Technologies at www.easyplasticmolding.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.
Step 2: Mount the original model into the first half of the mold frame.
Note: this mold frame had cutouts for sprues on all 4 sides, so I had to block the other three cutouts. But, I noticed that the epoxy mold frames on the www.easyplasticmolding.com website now only have a single sprue cutout, so this may no longer be necessary.
Step 3: Mix & pour the epoxy for the first half of the mold.
Then I carefully poured the epoxy mix into the mold frame to surround 1/2 of the tropical fish. Again, pour slowly to avoid creating air bubbles in the epoxy.