I've already mentioned my preference for a GA spoiler in another thread, but it would take a great deal of fabrication due to the drop-off trunk we have. At that time, we discussed the LE spoiler that Tbat mentioned being on Jason's car. If you look at it, it is a GA spoiler, just bigger with a forward curve and a center support. Remove the center support and it should be pretty darn close.
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'13 Mustang 3.7V6 - MT82
'00 GP SE (L67 Swap, XP, Headers, 3.25")
'98 GTP, '01 Bonne