That’s the wonderful thing about this particular simple equation, it gives you your main box volume that the driver reacts to and the average overall cross-sectional area and the length of the “port”/line, for what is a horn but a specialized port. As long as you have the average cross section from start to finish, you can taper/flare the line. Generally it would be something along the idea of hinging on the center point and making the entrance smaller and the exit wider. As for the fill, what my calcs say is to put that part of the fill in the line area to help slow / dampen the energy in order to reduce all that fractured energy that can happen in such a long outlet. And if course, you can stuff the main box as you see fit.
Just like anything else in this hobby, it’s a scientific art. And only through practicing our art can we refine the science needed to make the art that much better.