The cover on my C70 MKII is also missing. I've owned the car 3 years now and didn't realise it was missing until I saw this thread & dealer broke my obd cover thread. From what I can tell from my amateur delving, they are available on their own but it might be safest ordering from a Volvo dealers or breakers so you get the correct one for the car you are driving. I don't know if all OBDII ports are the same.
Fred
