Start with a 3D printed case, add some laptop parts from Framework Computer Inc https://frame.work/ as well as some other electronics and hardware and what do you have? A fully customizable, fully upgradeable, and fully repairable handheld gaming PC that you can make your own!

Able to use any Framework 13 Motherboard and Battery
8.9" IPS Touchscreen
Front facing speakers
Joycon docking/charging
Framework Expansion Card compatibility
Docking station connector
Power and volume buttons
Kickstand
Additional expansion provisions

The build process is outlined in a YouTube video https://www.youtube.com/watch?v=xQa__ukKvmM
I recommend watching it completely at least once to get an idea of how D-Pad is assembled.

You'll find all the information you need to build one in this repository.