A passive intake vent, sized for an 8mm cabinet panel. The core hole you cut is 76mm. Body measures 9cm by 11cm, moulded in PP plastic. It clips in by hand once the hole is drilled.
Why bother? A fan pulls air out. Something has to let air back in, or the cabinet seals up and the fan just labours against itself. One or two of these on the far wall fixes that. Fresh air comes in cool, crosses the plants, leaves warm through the fan. The plastic copes with cabinet damp and will not corrode.