Just curious, but nearly $20 for the foam ball seems exorbitant. What about using an 8inch styrofoam ball instead? Almost a third of the price: https://www.amazon.com/Crafting-Modeling-Decorations-Decorative-Styrofoam/dp/B0117XSEFG/ref=sr_1_3?keywords=styrofoam+ball&qid=1556744593&s=gateway&sr=8-3

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Not sure how the foam vs styrofoam would impact the sound.