Like most EFIS systems, Grand Rapids Tech's EFIS Horizon 1 system requires the use of an Outside Air Temperature (OAT) probe. I mounted one for the Dynon under the left wing, and so the right wing seems like a good place for the GRT unit. While the Dynon probe is a slick, polished aluminum cylinder, GRT's probe is merely a small white plastic cylinder with wires coming out of it. It comes with a pathetic little plastic clip, but that's not up to my standards. I decided to drill a hole in an AN816-4D fitting and mount the fitting through a hole in the wing skin. It'll be secured with an AN924-6D nut and a couple of big washers.
After I was done shoving the probe through the resulting hole, I noticed that the white dollop of silicon at the end of the probe had come loose. I replaced it with a dollop of blue Permatex RTV. I also filled the back side of the assembly with RTV, and made a fillet or RTV around the base of the probe, where it comes through the fitting.