Can't you go for a drive and let everything warm up and then measure the temperature in the area where the filter will be that might give you a better idea, and unless it's a very confined space I wouldn't have thought air starvation would be an issue, it's not a brabham BMW out for qualifying after all. Temperature would probably be the biggest enemy, in a poorly ventilated area. Measuring it will give you a better idea of how much ventilation you're getting.
By filling in the vents what exactly do you mean? Making it look like there was never a vent there or blocking it off so it still there but doesn't do anything. As you could open some of the vent back up if that was the case to let some air back in.