casting molds of plastic/rubber/fiberglass parts? anyone done it?

[JordanVw at aol.com]

im looking to reproduce a rear wing/spoiler for a vw that is no longer being 
made (rear spoiler for the vw mk1 cabriolet).  i have found an original  but 
the owner doesnt want to sell it, tho he has granted permission for me to make 
a copy.  the spoiler is not mounted, and is a black rubber/foam type spoiler 
like those used on the A2 jetta or audi 4000q, and looks similar.    i was 
thinking of reproducing it.. the finished product does not have to be done in 
rubber/foam, it can be done in plastic or fiberglass, whichever is easier to work 
with.  what is the best way to make a mold, without damaging the original?  
and what is the best bedium to cast it in?  fiberglass?   anyone have any 
experience with this?  

thanks

chris