Be a geek, make a letterbox

"And I think how beautiful it was then, through that void, to draw lines and parabolas, pick out the precise point, the intersection between space and time where the event would spring forth, undeniable in the prominence of its glow; whereas now events come flowing down without interruption, like cement being poured, one column next to the other, one within the other, separated by black and incongruous headlines, legible in many ways but intrinsically illegible, a doughy mass of events without form or direction, which surrounds, submerges, crushes all reasoning."
--Italo Calvino, How Much Shall We Bet?

You're surrounded by signifiers and you don't even know it. Anytime you visit a monument or public marker in DC, you're probably standing in the middle of a clue for a letterbox. For somebody out there, the number of buttons on Thomas Jefferson's coat is highly significant, because it represents the number of paces to take in a north-westerly direction to intersect with her next clue. In fact I count maybe 25 or 30 active ones right now. It's so cute - it seems that a lot of people make them as a way of publicizing their favorite historical figure.

I haven't made one yet, but I probably will someday. Any suggestions?