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How about a rotary type valve. Have your opening as normal but install a length of, say, 4" drain pipe cut in half along its length. Fit some end plates with a pivot hole so the half pipe rotates around its centre. Kind of like a ball valve but with pipe instead. That would overcome the wind pressure opening the door at speed. Not unlike the sliding door so whichever is easier i suppose. Just throwing ideas into the pot!
ollie
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