OK, sure, this is soon to be obsolete, with good 16mm boards showing up more and more, but still sometimes you gotta do what you gotta do.
I always find this kind of thing tricky. It can be done with some careful measuring and scribing, but I seem to screw it up all too often. You can try to align the two boards and clamp them, but they always seem to slip at just the wrong time, or something else goes wrong.
So solder them together! Solder paste, a bigger blob than you'd typically use for a bare emitter, heat, and if you have the right amount of paste, the lighter 16mm board will snap into place just like a LED does. Then you have a template in place that ain't going nowhere until you're done.
Oh, and one other thing: With the disposable AL board on top, you can clamp onto it with the vise grips and it protects the top of the copper goodies underneath.