Does someone know a method to prevent an accidentally deletion of files (write access necessary)?
A filetype-group would also be possible, because the files are all of the same type.
If you don't need to modify the files then the Read-Only flag is a very quick way to do it (assuming Opus is not set to ignore it!).
If you do need to modify them then you can use file permissions instead, e.g. remove or deny Delete access from your account. That may cause problems with some programs which edit files by creating new copies and deleting the old versions, though (e.g. Word).