Reminds me of the time I built a ghetto Switch out of a Raspberry Pi, a telescope controller, a powerbank and a touchscreen.

Sold two or so on ebay with the intent of earning enough money to build one for myself.

Until I realised I could have a much better handheld just by using that controller with my phone.

Anyways, there are many instructions out there to build a handheld with a Raspberry Pi and similar. But if you want to match the Deck in terms of performance and portability you'll need a custom PCB and some serious electrical engineering and soldering skills.