I bought an indicator alarm off Ebay - ultimately never fitted it because of the Roadster's canbus system. It wasn't expensive and looked easy to fit - just beeped with the indicators. I've just looked on my Ebay history but can't find it to point you at it. I think it was called an 'Audicator' - but Googling that doesn't bring it up either.
Fortunately the Aero's indicators turn off like a normal car - unlike the Roadster's which hardly ever did.
