If it was a permanent zone of truth, nobody would be able to do the one most important piece of this -- ie. know whether the person in it has failed their save and thus is speaking truth.

In reality I would expect the rich and powerful to simply have two or more priests cast zone of truth in the same spot.

The more zones of truth you have, the more people know whether the person in it is lying or not, and the less chance of corruption. At some point you do have to trust your clergy.

Alternatively, if you are the ruler of a kindgom it would be trivial to commision a magic item that allows you to cast it yourself. The spell is level 2, it should be easy to do. Even once a day, then YOU would be able to question your own clergy and know if they had done anything. Because it is literally foolproof, you would have no problems. Punishment for failure could be immediate execution, because you would NEVER have a false positive.

Using a magic item to cast a spell still counts as you casting it. A luck blade can give you Wish fatigue, and so on. Thus, a magic item that lets you cast the Zone of Truth spell would allow you to know whether someone failed it.