I thought I would share my Urban Letterboxing cautionary tale on a carvers' forum because I know you'll understand the work involved in carving two clues backwards.
This retired letterbox was called Secret Staircase. Letterboxing clues often take boxers to a good cafe, welcoming business, or a little rest area that's unexpectedly wonderful, so I thought a difficult-to-spot 5 story staircase that goes between two very different parts of town was a great place for a letterbox. The first stamp was in ivy at a very nice empty former bank, the second one was at the landing of the staircase and contained the logbook (hence the spoiler shield over the third stamp), and the third stamp was in a park. But this series had problem after problem.
The first disaster was that a rat started to eat the stamp shown in blue, so I spent forever recarving it and found it a sturdier box. The main disaster with this box was garbage and human waste, which finder after finder let me know about. When I heard about the garbage I amended the clue to ask boxers to spend a little time picking up if they are able, but when I heard about the waste I retired the box. I didn't even reply to the person who told me about it, nor finish reading their message. The last failing was just that the logbook was so big that non-boxers found it over and over, which was actually very sweet but is bad for longevity.
They used it to write to me, signed it, put in stickers or little drawings.
My subsequent plants have been much more successful.