I think the high shide have immunity against mortal magic, if it is the armor, it is an IoP, of course.
An amulet or a talisman should possibly be an enchanted item or a potion that puts up a block against hostile magic by the usual rules.
I would not set them up as a counterspell, because those are for interrupting long term magical effects rather than evocation attacks.
I agree with respect to the item mentioned from Warrior, the easiest way to 'model' something like that, is as a potion, a single-use item which after being used (successfully or not...) is consumed depending on the number of uses it is assigned. When it is used, it would put up a Block of strength X, depending on the crafter's Lore skill, any Crafting focus items used as well as Refinements.
The one area where others might not agree with me on, is that I would have the item set to 'automatically' Block cast magic, as opposed to something which the wearer has to trigger. Or at least that was my impression of what happened in the short story.
-Cheers
Remember that you can also (occasionally, and maybe at the cost of a fate point) make use of skills completely beyond their normal trappings. So, for example, we could try to use resources or contacts to defend against a magical attack, with the justification being that we bought/got some sort of defensive item form someone.
I'm trying to come up with tech that fizzles magic. One-shot items that absorb magic then get fried.
Something like the Thorn Manacles Harry was in during PG would probably have to be a (cheap) Item of Power, since they would effectivly be always on.
Something like Lord Raith's magic immunity would require a major threshold effect on himself. A Legendary threshold would outright stop the vast majority of magic and cut off thaumaturgy that didn't have a piece of him or his Name for sympathetic link as thresholds are magical borders. That kind of ability would cost -6 to -7 for a Legendary Threshold and would require either Outsider powers or the individual in question to contain lots of Mordite.