Cast Mordenkainen's Private Sanctum every day in the same place for a year. Among the effects you can choose when you cast the spell are:
- Sensors created by divination spells can't appear inside the protected area or pass through the barrier at its perimeter.
- Creatures in the area can't be targeted by divination spells.
The most important feature of the spell, for this question's purposes, is this line:
Casting this spell on the same spot every day for a year makes this effect permanent.
Now, obviously using a fourth level (or higher, if you want a larger area) spell slot every day for a year is not an insignificant effort, but there are no material components consumed by casting the spell, so it only takes time to set it up, and nothing to sustain after the year is done. It might be a worthwhile project if your character has a long period of downtime between his high-level adventures. You may not have many uses for your spell slots on boring non-adventure days!
The biggest downside of course is that it's limited to a single location that you pick in advance. If you need to be protected from divination everywhere you go, this doesn't help you. But if you're a high-level wizard, you probably have the means to teleport yourself across the world reasonably easily, so you might be able to tolerate doing your most secret stuff only in your own wizard tower (or wherever it is you chose to cast the spell so many times). And you can still cast the spell as a one-off if you need to protect some other place for just a single day, when teleportation isn't an option.