I was unclear about whether this saying bore a good or bad connotation. The Pnei Mosheh explains that it is good, meaning that someone who prays in his home is protected and assured as if he were safely surrounded by walls of iron. The Talmud then goes on to ask about R Yohanan contradicting himself, in an earlier perek he says that the place to pray is in the Beit Knesset. The answer the Talmud brings is that R Yohanan here is talking about someone who prays at home at a time when there is no communal minyan taking place.
It seems to me that:
(gematria 239) ברזל = קנה + דף
Iron can bear the connotation of a ready page, and a pen to write with. Your tefilloth inscribe the page with your prayers, and those prayers are an ever-lasting monument, a wall of iron, to HaShem.