As an avid adult LEG0-builder I've used a lot of different ways to keep the bricks sorted when open each bag on the bigger builds. 
While I found some nice options to print I decided to create my own which works well for me while keep learning myself Tinkercad. 

• Bigger tray with a big area for the main bag and then 2 smaller trays for up to 2 of the smaller bags that are usually included
• space for up to 2 optional smaller trays that fit inside the bigger ones.
• Lid to close the bigger tray when not in use or when you want to put it away without risking loosing any pieces
• Smaller low profile trays that can be used when picking out the pieces for a specific set

The bigger trays are stackable. Versions for both the bigger printers and the A1 Mini are included.