You can make physics based meshes placement on scene in few simple steps:

1. Drag BP_PhysSpawnerVolume on scene
2. Change settings as you want and hit "Fill volume" button in actor's detail tab
3. Hit Play->Simulate and wait while physics simulation is going on
4. Press "Save objects" in actor's details tab and stop playing in editor
5. Select volume again and hit "Load objects" buttons to place meshes on scene. Few additional buttons allow you to convert loaded meshes to ISM instances (you can work with them before it (change transform)) or load them as ISM instances directly.
6. This BP can use two meshes - one with accurate collision (for best physics simulation) and one with optimised collision (will replace prev. mesh).

Check video tutorial to see how it works

Video

https://www.youtube.com/watch?v=dWsx_qFYK-g&feature=youtu.be