You're right, it's a reverse Torx. Your local auto parts store will have them. I think it was a 16? I can't be sure, but yeah don't try any other socket other than the correct Torx. You'll round it off.
2009 AM V8 Vantage Conv - Berwick Bronze, Quicksilver Exhaust
2013 M3 Conv - (RIP) thanks to uninsured a-hole
2004 Porsche 911 C4S Conv