I retrofitted a Rennline cupholder to my 430 Scuderia, and thought to do the same on the GT. It's Rennline model SA05-14 (the one for the Porsche 944, and works with seat rails that are on the floor). It mounts on the seat rail bolt. I had to enlarge the bolt hole on a drill press, and had to bend the main tab down, but it fit pretty easily. It's been a great cup holder in the scud.