All you need is a supply of envelopes that are:
- Opaque (you can't see through them);
- Open at the narrow end (not a requirement but it works better this way); and
- (ever so) slightly larger than the cardstock (again, opaque) your spectator is writing on.
Making your own envelopes is a pain in the butt, especially if you do the version where you are handing them out as souvenirs.