This is a 12 by 25 cm backpack insert with 3 rows, the first being 5 cm deep (I use it for headphones), the second row being 4cm deep (I use it for laptop chargers etc. and a small row of about 2cm which I use for phone, external harddisk, pen or cables.

I recommend printing this in TPU to get a nice, flexible organizer.

It has been created for those open backpacks without any internal compartments for when you still want some order with your stuff and don't want everything to rub against each other all the time.