Now we need to punch a hole in the geometry for the vent. On the upper side, select the two polys on top and inset them as shown by the red polys. Delete the resulting polys shown in green. Move the blue vertices to the center line (i.e. make their x-coordinates equal to zero) then delete the red polys You will now have a hole in the top side of the geometry but not.
Do the same for the underside so that there is a hole in the upper and lower parts.
Let's bridge the holes. Create polys between the two holes as shown in red. You can extrude down and weld vertices or you can use the "Create" tool. Select the edges shown in green and use the "Connect" tool to add an edge through them for more detail. Do the same for the underside. Chamfer the edges around the hole to sharpen them.