It would be mildly impressive if a manufacturer made a fake cover with a switch to detect when it's "closed" and make the main camera stream filtered to look like the cover is real, while having a 2nd back-door unfiltered stream. Of course this could be detected by someone who took the unit apart.

On Thinkpads, the cover has a prominent red dot painted on it, that ends up in the location where camera lens would normally be. And, of course, you can visibly see the edge of the cover move as you slide it. So I don't know how you'd fake that.

Just pick a material that's opaque only in visible spectrum.