Here is another kit that lets you build a fun handheld game console. The Game Boy Null has a 2.8″ screen and 1000 mAh battery that lasts up to 3 hours. It is powered by a Raspberry Pi Zero W.
The kit comes with the PCB, buttons, screen, power switch, plastic bolts, and most things you need to get started. Sets you back under $50 at this time.