Im going to assume that you are going to simulate using an incompressible flow. The boundary box that you have defined can be used to set your conditions, inlet velocity, outlet, etc… no need to import. Here you can see that i have defined a wall of the bounding box as the velocity inlet, you can do the same for your simulation.
The cartesian boxes are used more to make local refinements in cell size around the geometry during the meshing step using the inside, outside, or distance settings. This is shown on the floor of my bounding box where the I used a cartesian box to refine the cells around my geometry.
The air assignment can be applied directly to the bounding volume of the mesh. In my example i have other cell zones that must also be included.
Based on the geometry you uploaded, i would suggest increasing the size of the bounding box. Typically the bounding box size should be about 8-10 times as large as the geometry so there are no problems with dissipation or back pressure from such a short wind tunnel.
If you have any other questions feel free to post or create new thread topics. Simscale has a very active forum to help with any problems that may arise.