If you are not disposed to burn out the plastic, you can make
an rtv mold of the plastic item, shoot a wax and then cast the
wax. RTV is room temperature vulcanizing . Its very useful
stuff: you can even take a mold from a wax with minimal
shrinkage.
Haven’t tried that but intend to soon!! Going to try the burn
out first I think, have seen other large pieces in plastic that I
would like to try later. Thanks …Jim r