I too am looking for a polarizer for the Horizon, for my GoPro I ended up gluing a camera filter ring to the case so that I could use standard camera circular polarizers, if you ever do this leave the filter off while the glue hardens, there can be out-gassing from the glue during hardening that can coat your lens and ruin it, I had to replace the outer lens on my GoPro because I used cyanoacrylate.
The advantage of a camera polarizer is that it allows easy adjustment of the polarizing angle/plane thru turning the filter which is critical for using a polarizer. Otherwise it will not reliably reduce reflections. Definitely one more thing you have to remember when you set up your camera…It looks to me like a different front cap for the camera with a standard camera filter ring size would be the most straightforward and effective solution, meanwhile I will experiment with other schemes of attaching a polarizer, duct tape anyone??
Also I have read about another approach sometimes used as a stopgap, and that is to buy a piece of black velvet to cover the dashboard with the intent of absorbing all or most of the light that falls there. Not sure effectiveness as I have not tried it.