Just to throw this out there: You cannot undrill/melt holes. Work your way up to what works best for your air flow and you will save yourself the ugly prospect of covering them up.
I have a reptile basics sweaterbox rack, each tub has 4 holes in it. During winter months these holes point in to increase humidity. During summer months they point out to decrease humidity. Works great for where I live, and there aren't too many or too few holes.
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