This very nice gauge mount is universal, but we specifically like it for the 2008-2014 Impreza/WRX/STI, which is otherwise pretty difficult to fit for gauges.

The mount consists of a small billet aluminum pad, which you bolt to any surface, or at least any surface that's accessible from the back. We like to mount it on top of the steering column cover. The gauge cup (also formed from a hunk of billet aluminum) bolts to the pad. A clever bolt system allows the cup to swivel to your desired location, and then tightens for a snug and secure fit. A small cap covers the bolt head. Included cushion pads (52mm) or set screws (60mm) hold your gauge snug.