A small plastic unit which fits in where the ashtray used to be. Holds two 12 oz. cans, sort of. Installs in about 45 seconds, no tools.
Only okay. If you don't smoke, like me, the ashtray is totally useless and this is an improvement. However, the second can is held at an angle and can thus spill if it's full.
I bought it from Advantage Miata in Tucson and they were great. It was cheap and it does come in handy on longer trips, but it's of questionable value in a car like this ('92 base model).