Ap/Utility Supports and Armor

What's everyone's opinion on a support item with armor. Preferably one for the utility supports (Lulu, Sona, Janna, Karma). Is there a reason there isn't really an armor support item yet? I mean the frozen heart I guess, but that's more for melee, and is expensive, and nerfed. I remember there used to be an item that was like the locket but gave armor to nearby allies with the magic resist, I'm not asking for that item specifically back because as I recall it was immensely strong, but I feel like the ap/utility supports need better itemization options when it comes to stuff like this.