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To allow air but not light you need to build a light trap. There are infinite ways to do this, but the basic idea is that light travels in a straight line but air can bend around corners, so build a non-linear path for the air.
What I have done is build a couple of "S" shapes out of pvc pipe and fittings -- about an 8 inch segment of straight pipe and a 90° elbow at either end. Spray paint the insides of these parts flat black before assembly to eliminate light reflecting and passing through. Cut a hole the same size as the pipe, put the pipe through, seal around with caulk, then put the elbows on. Attach a fan as needed.
For other ideas try googling "light trap."
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toastybiz
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