macOS – Use Clamshell Mode Without Power Adapter

It may be easier to use the "Sleep" option in the  menu (upper, right). It sounds like the "clamshell mode" you're talking about is actually "Sleep mode" to the computer. I believe that this is the most effective way to get things into "sleep mode."

Other options:

• Setup a "hot corner" that puts everything to sleep.
• Use keyboard commands for sleep. (Command + Option + Eject)

There are two solutions I can think of:

• SwitchResX
• Power plug adapter with remote control

SwitchResX has a number of features including "disable display" which should deactivate it until the software turns it back on. From their FAQ:

SwitchResX includes a function to remove a display from the System's list of managed display. This means the display in question, once deactivated, will not be visible from the System or from any application. Only SwitchResX will be able to activate the display again.

Another simple solution is to use a power plug with a remote switch. It's an easy, non-invasive solution that will get the job done without having to install software or have daemons running that could interfere with your display. macOS will not detect the display if the power is cut (though it will if it's in "standby" or "sleep")

Both solutions cost about the same (less than $20USD). SwtichResX has a 10 day free trial and the power adapter comes with a 30 day return. The software has the benefit of being elegant; but the switch, though a bit crude, has the advantage of you knowing for sure you display is disabled because it's physically off.