regenerating defensive shield item.

i use alot of mana based champs and i love seraphs embrace but thats just a one time use active item and orb of winter refreshes all at once. what i want to see is an item that uses mana per second and turns it into a shield that builds into a shield gradually instead of all at once. it should have a toggle on it so that it can be turned on and off so you can choose to conserve mana or be on the defensive. it will need a max size or a speed at which it decays based on the size of the shield that gives it a soft cap. another thing is that it can have as part of the passive that the champ gains damage reduction while there is a shield active on the champ.