Here is a video by Ben Heck showing how he made an Atari 2600 Jr Portable unit using the single-chip Junior model.  It's almost an hour long but quite interesting to watch.  Apparently, this project sat in a junk box for several years before it was found again in the fall of 2018.  He worked on it through 2019.  Check it out...

Video table of contents:

2:30 - restoring junk box IC
5:34 - setting up parts in Eagle and initial design
10:50 - testing cartridges physically with basic PCB design
15:30 - stuffing main logic PCB & testing
21:03 - mocking up the case design
23:49 - design philosophies for the casing
29:09 - material testing for 3D printing the case
31:41 - designing & stuffing the front half PCB with controls and charge circuitry
40:30 - final case construction and assembly
49:25 - game play montage with various cartridges

https://youtu.be/4EUMil5B3Os