Simon, if Lynn's scratch-built method looks a bit daunting, you could see if Peter Ransom of GT40s Australia will sell you his basic deep nostril kit (without the top bonnet hatch).
What you would get would be 3 bits - the 2 nostrils & a centre joiner piece. I am sure that they could easily be modified to fit most cars. I fitted this kit to my hatch & after some cutting & shaping, it worked out fine.
I think Peter's email is :
peter@drbsportscars.com
Kind Regards,
Peter D.