Would have to be very very low profile. There is not much room under that panel at all. I would put a really nice speaker in the stock location and then put a tweeter in that panel if I were you. Which is what I have seen done on all serious "systems" I have seen in these cars.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.