Yup. Soldering iron. Got mine for $5 at Walmart.
It doesn't leave any burn marks. It's really easy. Like slicing butter with a knife.
And I second the notion to do it outside or by a stovetop vent. The fumes from the melting plastic are pretty strong.
I do 5 evenly spaced holes on the left and right side of the tub. I get a perfect 60% without misting. You may want to try fewer holes first and adjust accordingly.
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