[Everett33]

Dry fitting a gusset to the stock material and scribing a line would be the best way to mark the cut line. My favorite way to cut aluminum is to clamp it into a miter saw with a carbide blade installed. You can line the material up with the tooth edge perfectly, clamp in place, and then cut. The carbide blade makes the cut quick and easy and only requires the edges to be broken with a file before it is ready for use.