These might help. The top picture is right side up. The bottom picture is upside down.
There is a lip on the back of the shelf above the shelf surface, but no lip below. I might want to use two or three fasteners along that lip, penetrating the lip itself, and fasteners underneath the shelf at the bottom of the lip there.
I wonder what fasteners to use, and how to use them. I could use something that has a screw head on it, but a button-head might look better. I would need to drill first and install it with a hammer, basically. Hm....
If I drill, then what kind of bit do I use? Hardi-Plank is semi-cementitious.
And then do I use a bit of silicone to waterproof? And/or do I use a rubber washer between the sheet metal and the siding? Don't tell me I am overthinking this. I already know that.
If I am to put a fastener through the sheet metal, then I will need to drill that. Carefully, so as to avoid bending the sheet metal.