Mat board won't approximate working aluminum very well. If you want to pratice find some scrap aluminum sheet, or else buy a peice of aluminum extrusion from a hardware store and have a go at that. But you might as well just go for it on your carrier. Just be careful and stop well before you think you've taken off enough and check the result.
A file will load up quickly working aluminum, you'll need to clean it often with a wire brush or a file card (a short bristle wire brush meant for maintaining files).
You could also just work with sand paper, which would be my first choice. 180 or 220 grit will work well for the initial work, then some 320 or 400 to smooth it to your desired finish.
The sand paper will be a bit slower than a file, but give you somewhat more control. If you want a very flat surface wrap the sand paper on a piece of wood or metal that is near the width of the side you're working. If you want to remove metal faster, 150 grit would work, but much coarser than that will leave a surface that may need a lot of work to smooth out, and it would be pretty easy to take it too far.
Sandpaper will shape even thick aluminum surprisingly fast. For a metal, most aluminum is quite soft, with great care, you can even cut it with carbide tip woodworking tools.