The weight of box mixes has changed over the years. They can range from 16 ounce to 18 ounces. You can use either or. This won't affect the outcome of your brownies!
Lining your pan with foil or parchment paper, making sure it's hanging over the sides of the pan, will provide "handles" that allow you to lift the entire block of brownies out of the pan so that you can cut them on a cutting board.
If you would like to bake and take these, use a baking pan that comes with a lid, like this one. Once the baked brownies have cooled, cover the pan and bring your garnishes with you to top when you arrive at your destination.