I know the plates that you are talking about. The ones that are screwed to the rings on top of the shock towers.
I'm really surprised that nobody has reproduced these. They may not sell many, but they would be a piece of cake to manufacture.
All they are is a flat piece of metal, bent at a right angle, with the corners cut off and dressed a bit.
Having said that... the fastback grills hook in on the outside edge. There are two tabs that fit into notches on the inner quarter window frame.
There is a tab on the inside edge that will keep them in place with the decklid closed.
Kind of un-glamorous, but it should work. They might vibrate a bit, but they won't come off.
Edit - Sorry for the hijack. Good luck.
[This message has been edited by Raydar (edited 07-25-2015).]