drawback to a sawzall is the stroke length, they need a lot of depth. jigsaw is better for shallow cuts. but a cutoff wheel is better.
here's the theory, using a straightedge, score the line that you want to cut with a utility knife,or a scriber, rusty nail or whatever. take your grinder with...
just make sure that you get the thin cutting wheels, some stores sell 1/8 thick ones and they suck IMHO, 1/16 work great, and you can get ones called zip-cut disks that are even thinner (3/64), and work great, but wear quicker and may break a bit easier if you manage to jam the grinder, but that...