Here is what I have.


Spell Deflection (Su) At 8 level when a Shield Master is the subject of a ranged touch attack spell, he can make a melee attack at his highest BAB as a immediate action against the casters opposing attack X times per encounter, where X is his Class level/4. The Shield Masters attack roll becomes his new AC. If the caster misses, the spell does not effect the Shield Master and is deflected off his shield to a square two squares away. To determine which square, roll a d20. If the result is over 16, the spell dissipated. Otherwise, the spell hits the square corresponding to the number rolled. The numbers start directly north of the PC and ascend in a clockwise order. By taking a -4 penalty to the attack the Shield Master can ensure that the spell does not hit an ally. If the spell does when deflected, it instead dissipated.

Spell Reflection (Su) at 15th level the Shield Master can reflect spells upon their caster, instead of deflecting them y times per day, where y is Class level/5. If the Shield master successfully deflects a spell he can instead choose to reflect it by making a ranged touch attack against the caster.

A bit wordy, but it should work. Let me know if I have left anything out.

I also thought about this.
Spoiler
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Shield Bastion of Spells (Su) When the Shield master is the subject of a spell that is a damaging, ranged area attack he may raise his shield and resist some the effects of the spell. This ability functions like Evasion in all other aspects.


Not sure how well balanced/flavorful it is.