It's thin metal. You can probably cut it with household scissors, or a utility knife. Worst case, use a cold chisel. It might not be pretty, but it'll work.
Make the cut at the bottom, not at the top. Smooth it out as best you can—maybe fold the bottom edge over the mag-well lip. Burnish it in place with the thin shaft of a small-blade screwdriver.
Don't use your wife's best sewing scissors, though.