Use a bandaid. Cotton pad in the middle, no adhesive to muck up the lens.
Overall, the Post it is probably the simplest and cheapest solution. Or for fun a bit of something almost but not quite transparent. Let someone watching go cross eyed from blur and trying to figure out what they are seeing. Petroleum jelly dabbed on the lens?
I’ve always favored sowing a bit if discord to mock anyone spying and to let them know you know.
These things are silly expensive for what they are.
