I tend to develop projects in parallel and some get priority over others depending on how interesting they are to me. This project was pushed back of the list for some time (I started it in the end of 2014), but I always try to publish my whole project files/sources sooner or later.
So here it is: the GitHub repository for the Snail Mail Notifier. It contains the circuits for the emitter and receiver, in perfboard, etched and PCB formats. There's also the code for both ends and some pictures. The current documentation on Hackaday.io is sufficient for now I think.
Fun fact: the compiled code on the receiver is just 2 bytes shy of the total available flash memory.