Empty file works just fine. As stated
here:
As a safety feature a blank file, a single '/' or a /<directory> needs to be in the redirect file otherwise internal storage is loaded.
When rockbox is booted from SD it shows SD card content as internal memory so pretty easy to identify without touching internal .rockbox installation.
Your problem is wrong redirect file extension. Your created rockbox_main.clip+.txt (see Ext column on image). It should be rockbox_main.clip+ (without .txt extension)
P.S. Also /<directory> feature is currently
broken.