If a magic user is willing to devote 30 percent of her xp into a device, I’d let her. From that point, her spells would cast as if she were one level higher. Damage dice rolls that came up as a 1 would get rerolled and opponents would save at -1 against her spells.
If someone steals it, she casts as a level lower and saves against her spells would be at +1. Should the device be destroyed, she’d lose a level.
Fair? Harsh? Too Generous?