The banner_box_all.php is a very different kind of banners box ...

This allows you to add multiple banners based on the sort order in the Banners Manager to display at one time ...

You said you are using: SHOW_BANNERS_GROUP_SET8

Do you have something defined in the Configuration ... Layout Settings ... for the setting for: Banner Display Groups - Side Box banner_box2

That is currently the configuration setting for that constant: SHOW_BANNERS_GROUP_SET8

IF that is blank then nothing will display ...

You could make: SHOW_BANNERS_GROUP_SET25 (25 is just to make it unique)

And make a define statement for you new sidebox instead of using the configuration table like the other sideboxes do ...