Are you referring to P13 of his Master Printing Course book? It looks fairly straightforward. The baffle has to be slightly bigger than the extractor vent on the outside wall of the shed. Essentially its a box with 6 sides. A top side and front side screwed together to form a right angle. A smaller bottom side and front side screwed together so the bottom front side fits inside the top front side. Finally two sides close the "box" completely.The more the two front side overlap the better the chance that light can't get round the corners. In fact as long as there's even a small gap air will get out. All sides need to be painted matt black and I'd seal the edges as well where each side screws to any other with a waterproof sealant.
Originally Posted by MOPS
At the college where I went on a darkroom course the same baffle arrangement was constructed in the passageway between the darkroom and the white light room and was big enough to allow the movement of one person at a time. If that worked and it did, the much great overlap in the T Rudman illustration will certainly prevent any light penetration.
If you use an extractor fan then you'll need a simple vent( in effect a square hole) in the door as well with similar baffles. If you suck air out then air has to get in to replace it.