The problem with bondo or glass cloth and resin is the shrinkage when it cures. You need to make the "plug" about 5 to 10% larger to account for the shrinkage. Epoxy and glass cloth the shrinkage is alot less. You also need to "wax" or apply a form of mold release like pardol otherwise you won't be able to get the molded item off the mold or plug. I ususally use either paste wax for hardwood floors or paste wax for cars. I apply a good amount rubbing it onto the mold then apply a good thick layer and hit it with a blow dryer set on high to smooth it out let it cool then apply what ever material you are useing.
I used to work for a wildlife sculpture studio my job there was the mold maker and I also poured the items. We used a product called WEP(water extendable polyester resin)