Looks like the modder has a good sense of portability. This NES on NES Cartridge modding is an excellent work that fully elevating the wide face of the NES cartridge and the unportable NES console into something useful and pocketable.
Check out the video for more detail:
This mod is done by Airz. The portable unit itself is not self-made and it’s actually a Made-in-China affordable PMP that could do a whole lots of task such as playing video, music, FM Radio, and NES games. Looking at modding done by Airz, it did has its own good sales value if he is keen enough make a silo of them and sell it on Etsy or eBay.