Put it in Library/ (but not Library/Caches/). That's exactly the behavior you're looking for, I believe.
There is no current safe place to save local data that should not be backed up to iCloud.
The mental model for iOS is that it stores only two kinds of data: discardable because it can be reproduced, and backed up because it cannot.
That ensures that all your data always is safe. You do not have to worry about losing data when your phone breaks down, or about forgetting to move some data when you upgrade.
I think this also is the reason iOS devices do not have memory card slots. It complicates data management; Apple does not think that is worth the extra complexity.