Thought I'd post one I saw in my 101 Boxster Projects book, you can see it in the bottom right image.
Personally, I think it's a nice touch compared to the flat black lettering the car comes with, but I don't like it as a decal as that one seems to be in the photo. It should be a raised emblem much like what is on the front trunk lid, but I don't know how feasible this would be with the rear trunk lid as there isn't a built-in space for it like there is up front.