I drill 1/4" holes in my hatchling rubbermaids for ventilation (in addition to the gap inbetween the rubbermaid and top of the rack). I haven't had a ball get out of a 1/4" hole. I think they'd have to be in around 30 grams are so to get their big heads out the holes. You shouldn't have a problem........especially with animals in the 100 gram range.

Regards,
Corey

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