Never tried the 'cat litter tray'
Had a beautiful A3 a few weeks back, Strong stench of cigarettes.
Valeting every single inch of the car to an OCD standard. Changed the pollen filter and cleaned the glass. Once it dried it was no better. Next I sprayed 5 bottles of Febreze all over the roof cloth and seats. Though I had it sorted and it smelled like new. But once the Febreze dried out, same story. Cut open onions and placed coffee granules inside the car which masked it a bit. Tried two of those vanilla odour bombs, waste of time!
Brought it round to a chap who has a Ioniser machine, left it with him overnight. next day the car smelled normal again. With all the time, effort and products used, I'd have been better off buying one first off.