Much like this french modder, Darkeru from the Ben Heck forums was able to condense the NES and controller unit to fit into an actual NES Cartridge.

http://www.blogsmithmedia.com/www.nintendowiifanboy.com/media/2008/07/portablenescart_cs_0705.png

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Deathsythe via nintendowiifanboy.com »