Problem is, if the vent openings face towards the cowl, and the under side of the hood is sealed air will draft away from the intake. Not that it would reduce the air intakes ability to pull, but you would be creating a negative pressure zone around the intake.
The main purpose of a rear facing vent is to draft cool air up into the engine bay and out thru the hood. If you seal an airbox to the hood with the vents drafting away there would be no real benefit, unless you had a duct coming up from beneith the car that plugged into the bottom of the air box. This would allow the air moving quickly across the hood to draft air from the vent, which would have been pulled up from the ducting under the car.