2 scripts, 1 item (scripts the same as Belt of Blur)
This gets around items being restricted to immunity to a spell school or general spell level. Make items that can make players immune to a spell school but only by a spell level. On equip, my spell example cloth makes you immune to 2nd level and lower Evocation spells.
The effect is supernatural (undispellable, not lost on rest).
Change the school and level by editing the local variables:
SPELL_SCHOOL = the number value of a SPELL_SCHOOL_ constant
SPELL_SCHOOL_LEVEL = 1 - 9
Warning: on unequip any supernatural spell mantle effects are removed (spells are not supernatural). Only use one item type (not rings) to use this. If there are 2 items equipped, when one is removed, the effects of both are removed.
I hope this lets you spread out item immunities. You will have to judge the additional cost.
10 - A Masterpiece, Genuinely Groundbreaking 9 - Outstanding, a Must Have 8 - Excellent, Recommended to Anyone 7 - Very Good, Deserves a Look 6 - Good, Qualified Recommendation 5 - Fair, Solid yet Unremarkable 4 - Some Merit, Requires Improvements 3 - Poor Execution, Potential Unrealized 2 - Very Little Appeal 1 - Not Recommended to Anyone