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I created a more professional look case/shell for mini version. Free filesand photos here: https://www.myminifactory.com/object/3d-print-case-for-acoustic-levitator-86542
I tested many sizes of styrofoam spheres, and if it is too big/heavy, it is pushed out. 3mm work just right for me. The density also is important. - Double check the phase of each transducer.