For armor, just have it modify the damage die. Light, -1 step, Medium -2 steps, Heavy, -3 steps. So if the damage is 1d8 + modifiers, it becomes 1d6 (vs light), 1d4 (vs medium), 1d2 (vs heavy) + modifiers.
Note, this makes modifiers a bit more important (that can be good or bad, depending on...