My RX-7 convertible has the same holder as
MartyP has screwed into the driver's side door panel- it has worked quite well and will even (though it's pushing it...) hold a Sonic Route-44 cup.
I have a plastic cup holder/coin tray with two cup spots (cup-coin-cup) that wedges between the passenger seat and the center console. Isn't very useful when I have a passenger in the car, but when I'm driving it by myself (most of the time at the moment) it works pretty well. I think I got it at Big Lots for like $5- it's pretty similar to
this from Autozone.
When I have a passenger in the car- they get to be my cup holder. :P