Define new subpanel layout not working

I am trying to define a new subpanel layout because I want no edit/remove button in a single particular subpanel. I thought I just add a new definition file additional to the default one under metadata/subpanels/default.php, e.g. MYNEW_READONLY_FILE in the same folder and just remove the definitions for edit/remove button.

and refer it in the relationship which should use this new file:

$layout_defs["<module_A>"]["subpanel_setup"]['<relationship>'] = array (
  'order' => 100,
  'module' => '<module_B>',
   #'subpanel_name' => 'default',
  'subpanel_name' => 'MYNEW_READONLY_FILE',
  'sort_order' => 'asc',
  'sort_by' => 'id',

This seems not to work. The panel is broken after a QRR. If I changed the original file it works (but then is applied to all subpanels). Do I have to define a new subpanel layout elsewhere, too ?

Copy the default subpanel file and then rename it and set the new file name. This file name will be new SubPanel reference.
in ‘subpanel_name’ => ‘default’, index, use this new Subpanel name.

Now it will work for you.