Easy to make man, base it off any aura and set aura bonuses to 0, but leave aura range. Than make an ability based of Runed Bracer's Magic Ressistance, and make a trigger:
Event
-Every 0.3 seconds.
Condition
-Unit has buff Spell Ressistance Aura
Action
-Add Runed'Bracer's Magic Ressistance to unit with buff.
And another trigger which will also set Resistance lvl to Aura lvl, and check if unit does not have buff Spell Ressistance aura and remove Magic Ressistance buff. Hope you got it
Add this;
Pick Every Unit in Playable Map Area matching Matching Unit has buff Spell Resistance equal to True
to make it affect all units within the aura's range and not just the hero, also ^ your trigger uses Triggering Unit, which does not have any effect in a periodic timer trigger.
Does that matter though?
Where do I add that?
Is it " Unit Group - Pick Every Unit in Playable Map Area matching Matching Unit has buff Spell Resistance equal to True" ?