Last August we covered a modder who goes by GmanModz on YouTube who built a portable Dreamcast they dubbed the 2DC. Now they are back with their newest project dubbed the Nintendo 64 SP.
This is essentially a portable Nintendo 64 that still uses cartridges and does not use emulation. While the engineering part is difficult to understand, we can tell you that it was designed in a similar design as N64 cartridges using a 3D printer. The hardware itself is a mixture of cutting down an N64 motherboard along with some custom PCBs and a VGA mod chip.
You can view a video showing off the handheld along with an overview of how everything was done down below: