This is the first thing I've uploaded here. Please let me know if there's anything I can do to modify it to your needs.

I made this to fit on the back of a 2017 Pacifica chair originally, but I had a few requests for the .STL and decided to make it universal for different headrest widths (or so I hope). I tried making it as printer friendly as possible as well.

NOTE: The dove tail slot has a small tolerance so it may take a bit of pressure in the right places to get it on 100% of the way.

The first two files are the original design, then I went back and changed a few things to the bracket and organizer itself (mainly rounded corners where it needed). The third organizer file just simply has a oval hole in one of the sides to save a bit of filament. I was going to make it more of a hexagonal hole pattern around the 3 main sides, but gave up when I realized I was lazy at the moment of posting this. If I get a free hour or two, I'll definitely go back and do that to have it save as much filament as possible without losing design structure and having stuff fall out on bumpy rides.

I'm also planning on making phone holder attachments for the front so you can set a phone up like those headrests that have tvs on the back of them. When I finish that, I'll add it here.

Overall, the cup holder part works great. It holds snuggly a standard large size fast food cup as you can see in the photo and there's room for extra stuff to the left.

Please let me know what you think!