It's likely that the dam produces a vacuum inside the airbox at speed
I was afraid you'd say something like that.
I've been thinking about redesigning the air box.
I'm planning to seal up the left side of the box, use a cleanable foam filter that operates in the normal (dirty air on the outside clean air on the inside) way.
Then opening a hole on the top of the box under the seat. Probably the best place for relatively still air. I have 15mm between the bottom of the seat and the top of the airbox