I wasn't aware of a "no digging" suggestion either: we've placed a
couple of boxes this way and have found several more in shallow holes
covered by a flat rocky lid. Although I have been called to task in
the past for going off-trail to get letterboxes out of sight, I think
that a little bit of digging doesn't hurt (of course that depends on
the local environment). Major excavation a-la those big scavanger
buckets the treasure hunters use may be another story...backhoes are
probably not a good idea.

Jay in it's-too-darn-rocky-to-dig-much-anyway CT