How to craft it
Work Benchspawning the mimic
The setup has several strict conditions and it silently does nothing when you miss one. It has to be a chest, not a barrel or any other container. The key has to be the only thing inside. And you have to actually close the chest, because the spawn fires on close rather than on placement. Jumping straight from that chest into another interface does not count as closing it.
The chest is consumed along with the key, so use a cheap wooden one instead of something you hauled down there.
Pulling a key into a chest with wiring will not work either, so there is no automated farm to build here.
where to do it
The mimic does not care what biome it appears in, so summon it in a prepared arena instead of fighting it in a corridor. These hit hard for the point in Hardmode where you can first afford to make the key, and having room to move is most of the fight.