In reply to Daniel James:
The packers are just thinner bits of wood - ply is fine, but anything will do - it just spaces out the back part of the hook to the right distance from the rear of the fingerboard mounting panel. (you can see what has been used in the 4th photo) Think of them as flat wooden washers perhaps. To be honest, you might not even need them, depending on how thick the bulkier bits of wood are that you're using. All that they're doing in that 4th photo is stopping the bottom of the board rotating inwards under load.
Yeah, just screw the timber together. Long enough to get a decent bite into the timber, but not so long that they'll come through into the fingerboard itself.
To join the fingerboard to the backing structure: the FB will have come with screws. They go through the holes pre-drilled in the FB and into a backing panel ie the bit of ply you'll need. This should be a bit bigger all round than the FB itself. Then round the edge of the mounting panel, you screw in the thicker parts to form the hook 'element'. On the photos you link to, it looks like the guy has maybe just used screws and clamped the top part onto the powerbar.