My 10 y/o son has had this idea for a cache for a couple years and it always felt improbable. Then coming back on a walk from Back Bay in the rain one day, we saw the perfect spot. After a couple days in the workshop, a ton of dust, one unfortunate break, a little glue, and a lot of effort … we made it work.
Beware muggles, high traffic street for cars and walkers.
Please replace the inner cache back in its vessel so it closes completely.
Happy hunting!