If you own a GC similar to mine (I've got a '99) you know the cup holder is a piece of engineering junk. it won't hold a cup of coffee or water bottle up while going 10 MPH down a straight, smooth road. I had the same problem, here's my solution.
4" length of 3" I.D. ABS plastic pipe
Cut one piece of pipe 2 1/2" long. It fits snuggly into the drivers side hole. If you only want one side to hold a cup you're done.
If you want the other side too like me, it's a little trickier since the hole on this side is smaller and the pipe won't fit in it.
Cut a 1" length of pipe. Cut a section off one side at an angle so it will lay up against the other pipe and sit directly over the other hole (see picts). Then taper it from 1" to about 7/8" on the cut edge (again see picts). This is because the housing it is over is rounded and the taper is to follow the rounded top face. Glue the pieces together with Gorilla Glue. When it dries file the glue down so it's smooth. A tapered coffee cup won't fit in the passenger side yet though, the top edge of the first pipe is in the way. So file it down enough to accept the tapered cup (once again see picts). Once this was done I painted mine with flat black paint so it would look a little better. Voila, cup holders that actually hold a cup for little or no cost and they work great.
The picts with a flash show a lot of detail of the filing and such, but in reality this is really not noticeable. Maybe one day I'll smooth things out a bit more.