Then I made a card with slits on it to hold the magnet. I ended up adding a little note saying "It's a magnet" because both of us felt that it wasn't obvious with how thin the magnetic paper was. The outside of the card is stamped and embossed and measures 5.5" x 4.25". A perfect fit for the envelopes I have.
The stamps I used are part of a Fiskars set.