Ventilation is important since you want a source of fresh air to breathe. You would want the out going air to be on the other side of the chemical trays from the incoming source and you being on the incoming side. If you can't construct light traps, a simple and inexpensive light "seal" can be made by getting black opaque plastic sheeting at a garden shop or Home Depot. Fold it in a couple of thicknesses and attach it to a dowel stick on top and one on bottom. Place a couple of "L" hooks above the door. You just hook the dowel above the door and extend it enough to make a light trap and the bottom dowel holds it down. It has worked for me for over thirty years. You can also block out windows as well. (If the sides need to be secured, use Velcro that comes with adhesive.) The whole setup can be removed when not in use.