Use PVC pipes for your intakes, with one 90 degree bend, spray paint the inside of the PVC black to absorb light, and rubberband some black pantyhose over the PVC intake opening to further absorb light, and that should do it.

You can turn off the fans and keep it sealed up, and the plants will likely be okay. That's not optimal, but they won't suffocate as long as you have the fans on half the day. They might end up somewhat smaller and less healthy than they would if they had fresh air supplied all the time though.