As a picture speaks a thousand words ( or in this case, a few pictures )  here's the process I used .......  I used CS25 Silicone rubber to make the moulds after I had made the original plugs.  I got shrinkage from the Silicone,  I then cast my 'Vac-Cast' Epoxy resin into my silicone moulds  ( got more shrinkage from the vac-cast resin ),  then I vacuum formed the parts I needed  ( headlight mounting brackets )  got more shrinkage from the 3mm ABS I'm using ........  the result is,  my ABS plastic headlight mounting brackets are 2mm too small all around their circumference !     >>