Nice cab. Here are a couple of ideas to address airflow:

IIRC, you want the total size of your intake vent(s) to be 1.5x-2x bigger than your exhaust vent.

The intake vent(s) should be as low as possible- I actually drilled holes in the bottom of my cab, which is raised off the floor.

The exhaust vent should be as high as possible- I actually drilled the hole on the top of my cab.

Also, it is best the the intake and exhaust are on opposite sides of the cabinet to foster the most air movement and to drag the new air through as much of the cabinet as possible.

Since this is a veg cabinet and not a flowering cabinet, you may not need to worry about total darkness when the lights are off (assuming you are not going on a 24/0 light schedule). Buuuuttt... you can drill 2" holes for the intakes using a whole saw and use 2 BLACK pvc 90 degree elbows put together to form a light trap, to reduce any ambient light that may creep in (stupid sprawl and light pollution).