Without additional support at the front, the shelf is in danger of sagging
in the middle. I would support it with a length of 1.5" steel angle, going
right across the alcove with the ends supported by the side battens. If
desired, you could put a lip downwards from the front of the shelf to hide
the angle.

Where can I get this angle from? Do they cut to size or would I have to do
it myself?

Marcus