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Most of the designs I've tried have been too large for small hands (like my grandkids), so I designed my own. I also haven't found a cap I like either, so I've got one that presses in and is pretty stable but I'm not seeing a way to upload multiple stls. The balls are 5/8" McMaster Carr #96455k58 (https://www.mcmaster.com/#96455k58/=1888jgl). Use a heat gun or hair dryer to warm up the plastic and they should pop right in.