I found these just now. I can't remember if these are the ones that the Startech XF used, but they look different, in a good way.
http://www.ebay.com/itm/REAR-BUMPER-DIFFUSOR-CROSSFIRE-coupe-roadster-NEW-/230582385030?fits=Make:Chrysler|Model:Crossfire&ha sh=item35afc7f986&item=230582385030&pt=Motors_Car_ Truck_Parts_Accessories&vxp=mtr#ht_532wt_932