Atari Age user lovablechevy has created a Pocket Atari 2600. In his own words...
Quotecompleted March 15, 2016
so, the Pocket Atari was actually my 2nd planned project, fully planned out between 8-9 years ago by now. lol! i'm just now finally getting it finished.
the Pocket Atari is an Atari Flashback 2 inside of a standard Atari 2600 cart, with controls, batteries, screen, and speaker. it is THE smallest portable i own, and literally fits in a pocket. hence the name.
it was a very challenging project for me. there was no painting, no bondo/filler/ABS/epoxy. i couldn't hide my flaws. i had to make perfect cuts. the internals also had to be wires of exact length and placed extremely well. this was a very very very space specific project. you'll be able to see from my very first picture here (my blueprint) just how detailed my planning has been for this. i didn't even want to attempt it till i knew, on paper, it was even remotely possible.
i was told years ago by the longhorn engineer that it's not possible. he attempted this project himself. well, i have now successfully proved him wrong. 
https://youtu.be/Noh-UMvqUtc (https://youtu.be/Noh-UMvqUtc)
Specs:system – Atari Flashback 2
screen - 2.5" "zate" screen with audio amp
power – 2 3.7v li-ion 700 mAh batteries (3.5 hours at full vol)
case – frankencased Atari 2600 cart
controller – buttons are from a wii controller board is from a Super Joy III controller
random tidbits – case has custom made sticker, custom single player paddle port, and difficulty switch
A few measurements of the system:
weight of portable – 4.6 ounces
thickness – 13/16"
height – 3 13/16"
length – 3 3/16"
More details, pictures, and pricing info here:
[size=150]Pocket Atari - lovablechevy's Flashback 2 in a 2600 cart[/size] (//http)
It is impressive indeed. Quite a way to prove someone wrong too!