I assume that you want a panel jack to allow you to plug the Garmin handheld in.
You can install a DB-9 connector onto the panel, and wire a mating connector to the end of the Garmin-purchased data cable.
Another technique that I have seen is to use a mono or stereo audio jack on the panel to handle the data lines and ground, with a mating plug wired to the Garmin data cable. (This is done by Rocky Mountain Instrument for example).
Independently of the Garmin - ADI connection, you may have other instruments that need serial data links (such as the Dynon D10A, engine monitors, etc.) to allow for data logging or software updates. Rather than have a connector for each instrument, you can have a toggle switch or rotary switch to select the instrument you want to talk to, sharing one connector.
In the big scheme of things, I kind of like the idea of the audio jacks -- they are small, need only one hole to mount, and are cheap. You can sprinkle them over your panel to connect into any instrument you want.
Vern Little
www.vx-aviation.com