Ars Magica has a lot of scope for magical gear for Magi, but what about a useful mundane item?
Is a switching circle possible? Its a carabiner device which can be used during the casting of circle or ring spells. Device is closed and locked, but can be opened as soon as an effect needs to be cancelled. The ring/circle breaks and the spell ends. The stability of the circle is far stronger or reliable than drawing the circle in something like salt, or scribbled with a stick.
These could be designed into floors or carried in the caster’s palm so that they are portable (if the saga allows such things, portable circles are sometimes controversial).