Magic Resist Items

I am an avid fan of being a facetank, Mundo, Nautilus and Leona are my babies. But when i go against a heavy AP team, i find i build 2 Spirit Visages, or 2 Banshees. I think that another flat MR item would be good for the game, as there are so many options for Armor, but so few for MR that aren't related to damage. It doesn't have to be fancy, but something like Thornmail, just pure MR, would help me out against constant AP damage.

An Idea I had, probably not balanced

Wall of Glass +80 MR +30 Armor UNIQUE PASSIVE: If a spell damages you, deal 25% of the damage you take back to the caster as Physical Damage

no idea on gold cost, probably about 2900

This doesn't insta-burst casters down if they combo you, since it does the damage after it is heavily reduced by the MR the champion has built. But, it will definitely kill them if a champion deals too much damage to them, and the tank can heal/shield/tank enough punishment.