I finally carved a stamp for Letterboxing, using a small piece of Speedball Speedy Carve. (online guides recommend keeping it small because some of the books found in the boxes are tiny.)

Photo of a small, hand carved block of Speedball Speedy Carve and a piece of paper with the image that was transferred onto it for carving.  The design is a Masonic Past Master symbol (square, compasses, and quadrant) mingled with a stylized banjo.  Under the symbol are the words "Arcane Banjo."

Photo of an impression of the finished stamp in black ink, next to a U.S. quarter dollar coin for scale.  The design is slightly larger than a quarter.