Depends a lot on what the intake looks like- if yer using duct already, a 90 degree bend facing down will do the trick in most cases. Remember to allow for drag when figuring intake size.

With more square openings, a flap of panda or other light-proof plastic over the intake ( oversize by a couple inches, tape to wall only over vent area, wieght bottom flap slightly) ( You make it long to allow ends to distort and absorb flex) works well in many cases.

You might also want to google up a diagram of a "darkroom blackout vent" . ( Ya didn't hear it from me, but this design works jut fine when made out of a card-board box.

Again, allow for drag from blackouts- you should prob toss on another 10-20% as a load factor in the fan-sizing and intake sizing math.