You could extrude the edges of the open body shell away at an angle, creating 8 surfaces that would be set to Internal in Fluent, which means that air will pass through those surfaces.  The faces that you show now will be walls. Then the solid body that creates the air for the "wind tunnel" can subtract the solid that is this extrusion with the open body faces at the end.

Here is an example. Attach your .wbpz archive if you don't understand and I might help.