The exit hole size and shape is best if it is made to create low pressure . louvers would work better than the flush cutouts you show.
The bottom of the plane is NOT a low pressure area so the exit holes should have some sort of angled edge/plate at front of the hole - to create the low pressure.
Air is not being blown in from the front!!!
It flows only when a strong low pressure area is at end of tunnel.
The air speed thru the tunnel will be constant -if the effective space remains constant.