These little air ducts attach to the undertray of your Boxster/Cayman and channel air towards the engine/transmission area. These are often missing or get torn off due to road debris while driving.
This ad is is for one of the two rubber flaps and includes the attachment plastic rivet.
It is old and the rubber is dirty to handle but it is still flexible and will work perfectly.
I believe it will fit either side.
Picture on request.