I use a blend mount on my C7 Vette. Attaches to mirror and I tap power from the mirror. Where I live, Washington, it is illegal to attach things to the windshield. Technically of course.
Something like this works great. Just gotta find the right power wires at the mirror. It has pins you push into the existing connector. Super easy.
I normally like to tap into an ignition on fuse but IIRC that the Rivian does not have a fuse box? In that case, @IPTV65's suggestion to use a blend mount wire tap to the mirror is also good, but not sure we have anywhere near a wire diagram to identify hot power.
IME it's a good idea to always have the countermeasures fire up everytime you turn on the car, and off when car ignition is off. I've learned that the hard way as there is otherwise no other reliable way for me to stay in the speed limit.
I haven’t found a good option yet. I use blendmount mirror mounts for all my cars but of course they don’t have one yet for the Rivian. I am providing pictures and measurements in hopes of getting them to produce one.