We studied several actual passport stamps when making the design. It has a few official boxes to tick with a (different-colored) ink pen, ostensibly the continent of the meeting. The @ line on the side is to indicate the location or event. Original Illustrator files are in the art folder of the SVN repository.
Making a stamp was cheap and easy. We exported Illustrator artwork to a high-resolution .PNG image, and sent it to an online stamp maker. The boxes on the HSPP are not the same size, but we found that 1.5×1.5inches is a good size that won’t encroach on neighboring stamps. Two laser-cut 1.5×1.5inch stamps cost $10 each. There’s a bunch of online stamp makers with about the same pricing, we used RubberStamps.NET because it’s next door to our Iowa office and shipping was super fast ($2 local, $6 worldwide).
Thanks to Tayken for design advise!