Write a short one and pin it up. Everybody who comes here sees the same board. No names, no accounts, and no idea where anybody is.
There is no location on these, coarse or otherwise. The drawer is called Postcards from Nowhere and it means it: the postmark is one of ten invented places you pick yourself, held in the database as a small number. Nothing about where you actually are is asked for, looked up or stored. The one thing kept alongside your card is the same anonymous random string every shared drawer in this cabinet uses — invented by your own browser, never derived from your address, and never shown to anybody.
The stamps are drawn for this page. Six of them, made of simple shapes with a real perforated edge punched round the outside, so they are nobody else's artwork.
One card every three minutes per browser, no links, and 240 characters — the same rules the guestbook and the bottles run under, for the same reason. It is a shared wall, not a moderated one, and it does not pretend otherwise.