Interesting thought. If speed and surface heating isn't a factor, then you could use a variety of other materials for drone and some other aircraft construction. I don't know about reflectivity for most materials, but from way back when, I remember something about traffic radar being less effective on Corvettes - perhaps because of shape and fiberglass bodies. The big engine blocks still gave them away. I really don't know if that was fact or fiction.
The early British Hurricanes had fabric covered wings, and the Mosquito night fighter were made of wood - seems like plywood. Wood, fabric, plastic, or fiberglass might have a smaller return than normal metal aircraft skins. Still the balloons that made the news were detectable once the radar discrimination circuitry was adjusted to account for slower moving objects. You'd guess that had to do with the solar arrays that some carried. I'd bet that all that has been covered while developing stealth aircraft. I did meet a guy who worked on stealth years ago, and jokingly said something about never feeling comfortable with chemistry or antenna theory - like the antenna guys probably wear conical black hats and capes with moons and stars. He said something like I jabbed him on both of his fields of study. I'm respectful of people working in classified stuff and didn't ask anything about that.
