Trim the flank steak of any excess fat, place in a zip-top bag, and place it in the freezer for 1 to 2 hours in order to firm up. Remove the steak from the freezer and thinly slice the meat with the grain, into long strips. Place the strips of meat along with all of the remaining ingredients into a large, 1-gallon plastic zip-top bag and move around to evenly distribute all of the ingredients. Place the bag into the refrigerator for 3 to 6 hours. Remove the meat from the brine and pat dry. Evenly distribute the strips of meat onto 3 of the air filters, laying them in the grooves and then stacking the filters on top of one another. Top these with 1 empty filter. Next, lay the box fan on its side and lay the filters on top of it. Strap the filters to the fan with 2 bungee cords. Stand the fan upright, plug in and set to medium. Allow the meat dry for 8 to 12 hours. If using a commercial dehydrator, follow the manufacturer's directions. Once dry, store in a cool dry place, in an airtight container for 2 to 3 months.