How to find the native DLL output directory in IPostProcessBuildWithReport?

During a player build I need to copy some extra files into MyProjectName_Data/Plugins/x86_64 (or the equivalent directories for Mac/Linux), i.e. the directory in the built player tree where the native DLLs are copied.

These are some settings files and such that my native plugin expect to live in the same directory where they were loaded from.

I can hack something up by looking at report.summary.outputPath and then doing some (windows/Mac/linux-specific) modifications to that, but I’m wondering whether there is a better way to get this information? I looked at the various classes under UnityEditor.Build but I haven’t been able to find what I want. Any hints anyone?