Ideas on a post card please

O’s Dad is a jack-of-all-trades. However, his skill lies is woodwork – detailed woodwork. We have asked him to make us a special box that we will use for our wedding photos. We will also use the box to keep our guest messages in. We are supplying our guests with pens and reference cards and hope that they will write us a message of good luck. We need some sort of sign that tells them what the deal is i.e. take a card and write us a message. But I’m not sure how to write the sign.
The best I’ve come up with so far is, which we might go with is:

“Guest Book – sort of” or maybe "take a card and write us a message"

Do you have any ideas or have you seen any inspiration? I don’t particularly want a long explanation just something reasonably short and snappy.

Thank you!