The ASUS ROG Ally is a super-popular gaming handheld with a 7-inch 1080p touchscreen, but someone has modded the handheld and added a secondary display. Check it out:
The ROG Ally 2nd Screen project on Github runs through the entire process, adding a second screen to the ROG Ally involved a 7-inch portable display, a USB Type-C dock to connect it to the ROG Ally, and then cramming everything inside of a few custom 3D printed parts.
YesItsKira used the WIMAXIT M728: a 7-inch 1024 x 600 resolution IPS LCD touchscreen display, and while normally you'd need to use an HDMI cable (and port) and a USB cable (and port) for video, touchscreen abilities, and power, but the ASUS ROG Ally doesn't have an HDMI port, which is why the USB-C dock was used.
The 3D printed parts include a replacement for the back cover of the ROG Ally, a shell that houses the display, and a 360-degree hinge that positions the screen above the primary display, allowing you to fold the second screen forwards or backwards, which is a nice touch.
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The modder removed the printed USB-C dock's PCB from its original case, stuffing it behind the display, and fitted inside of the 3D printed case. If you don't need any of the ports, you can save some space on the screen's PCB if needed.