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Use screen for porches and doors from a home improvement store and just staple it to cover the open back and I put egg crate over my overflows. Not even the snowflake can find a way out of the tank or into my overflows any more. Just don't use metal based screen or you'll be replacing it shortly from rust.
It took me losing 3 sets of clowns and a couple others before i invested in screening. So far, so good. Definitely gives me peace of mind. Well worth the cost.