EzekielRaiden
Follower of the Way
That's what I would recommend as well. It adds a mild complication (you must be holding the wand/implement in order to use it), and circumscribes use by number rather than time--it's unlikely you'd get more than 10 rounds of combat where the charged spell would be useful, but the wand can endure between combats.I would change it to create a temporary wand with 10 charges. Each casting costs 1 charge. That will prevent the inevitable abuses of 600 castings of a spell.