I’ve heard this phrase a few times in game desing- “Spell in a Box”. Might not be a great item to repeat a ton of times, but how about making one one time?
Spell in a Box
Magical masters often tire of their servants failing them time and time again. This box is the result. Any spellcaster may cast any spell into a Spell in a Box paying all costs at the time of the spells casting. The spell remains active for a number of days equal to the spells level. At anytime before the spell dissipates, anyone can use the Spell in a Box as an action to cast the spell. When the spell is cast from the box, the spell uses original casters proficiency, ability modifier, and other bonuses to cast the spell, but the creature using the Spell in a Box may determine the effects, range, targets, and any other choices made when the spell is cast.