there's this ... you would have to remove the uprights and wing, but would have a tailbase/ducktail when done. it would have to be slid down the trunk about two inches and probably trimmed a bit. i *think* extreme is a branch of gt3tek so quality is ok, and they could probably customise it to your needs (ie, delete the wing and reduce the cost considerably).
https://www.google.ca/url?sa=i&source=images&cd=&ved=0ahUKEwiXma73yvzJAhVB8mMKHWrzC-8QjB0IBQ&url=http%3A%2F%2Fwww.ebay.com%2Fitm%2FPorsche-986-Boxster-to-997-GT3-RS-style-Rear-Wing-New-%2F180397070920&psig=AFQjCNFZp9r0E-ZqZVjW96SKHUIbHjU6uA&ust=1451324321887338&rct=j