Thanks for all the advice,

Have checked into a lot of your suggestions already.
Picked up book that I think will work fine. It's thick enough for a LB to
be put inside and it has a dull Red cover. It's perfect, an old 1959 law
book to be filed in the Reference section. Will ask for permission first.
Hoping that it does not have to escalate to another level, if they say
no. Only will sneak if I have too. Would like this to be a permanent
letterbox (book)

Budster of the Catskills