[NorthSaber]

Our magic systems are actually quite similar - I too use Modular Abilities based on certain aspects or "colleges", and if a 'spell' is more expensive cp-wise than the Modular Ability could handle, the cost is reduced by taking a higher fatigue cost or casting-type limitations such as preparation required.

Basically I'm using Spirit Trapping (from Fantasy) which can be used either as fetishes (with gadget limitations) or ritual scarring/tattoo. Here's a sample write-up:

REMNANT SKULL (-30%): Modular Ability (Spirit Trapping, focus) 15 point pool (Physical also, +100%; Limited Use 4/day, -20%; Takes extra time 2, -20%; Costs 4 fatigue, -20%; Requires will roll, -5%; Breakable DR 4, -15%; Not repairable, -15%; SM -5, -10%; can be stolen by ST vs ST, thief cannot use, -15%) [40].

Note the +100% for Physical - most fantasy-type spell effects count as physical, such as the DR in your example. The will roll required is modified by Spirit Talent and gets a bonus from Spirit Empathy, but note that I don't allow characters to simply buy a high Will (haven't decided on the exact rules for this though).

I'm also not quite sure yet as to whether I should apply some limitation such as your Magic based -10%... Theoretically the bound spirits could be released or somehow manipulated, but I think this will be rare enough to be used more as a plot device than an everyday possibility.