A brass plug I would think would be best. I am sure lots of guys here are totally qualified for this. Maybe they will give some advice.
I would try to machine a plug that fits when its ice cold and the mount is nice and hot, then press it in. That's the only way imo you would get a nice positive seal. Make the cylinder 3/4 iches long so when pressed it remains straight. make a wooden plug and do some test fits with a dual remote setup. Perma Cool mounts are very reasonably priced.
I think you should go for 2 singles and then put them in different areas where they can be squeezed in.