How much head room

Quick question. I'm building my own rack system. How much space do I need to leave above the tub for air flow? I'm thinking 1/8 inch.

1/8-1/4 inch. If 1/8 then drill holes in the tub for additional airflow

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I use 1/8 and air holes but dont drill. I use a soder gun and melt them so no sharp edges