Recommended way to init non-Send data that's needed in audio callback?

What's the recommended way to init non-Send data that's needed in the audio callback?
I.e. initializing it in the audio thread but only once, before the loop starts running.
And without the overhead of thread-local storage.
This init code is also taking a long time (loading a bunch of VST plugins), and I don't want to block the audio callback on the first call.
