How to turn off an iMac screen when using an external monitor?

[tsinsf]

I have a new 2020 27 inch iMac, but I would prefer to use my (connected by display port) Nec monitor for photo editing. Is there a way to turn of the iMac screen, similiar to clamshell mode with a Macbook? This function in Switch Res X doesn't work in Big Sur! Any work arounds would be appreciated (other than throwing a towel over the screen or turning down the brightness).

[BobShaw]

I don't know if it is possible, but doubt that it would be a good idea anyway. Some things always happen on the main screen, like log in and notifications.
Personally i love the benefits of a 54" screen you get from two monitors and most programmes let you have browse on one screen and edit on another.
I actually use my 2013 iMac screen more than the Eizo anyway.
If you must do it then just put the second monitor in front of the first I guess. (:-)