I like the idea of demonstrating concepts like this with pencil-and-paper. Seems in this example, we could even skip the carbon paper and just use hole punches. For example, put an all-black grid on the paper and aligned with an all-black grid on the envelope, and then encode the signature on the paper which is inside the envelope by punching a series of holes -- imagine a qr code -- through the envelope and through the paper inside.

It looses the cool effect of the carbon paper though!