I've got one of those 12" angle finders for cutting trim - I've done stuff like this with it, like measure at each end for the height of the rail, and run a string or tack up a straight scrap of wood. Find the angle for each end. For something short like this, I might first cut from plywood or a 2x4 to make a template, cut a little fat and shave it bit by bit on the chop saw til it's a perfect fit, then transfer to your actual lumber. Many ways to skin a cat, I'm not a pro but I really enjoy designing and installing trim; windows and doors are more my thing though.