Bauer Gridfinity Inserts

Simply print all 4 sections and slide them together inside the unit. You can glue them together if you'd like but the dovetail and engagement on the interior features of the box does a pretty good job.

The space inside the box is 15u tall so you can fit either 3x5u gridifinty bins high or 10u+5u gridfinity bins in the vertical direction. (A u is 7mm)

Link to the toolbox from Harbor Freight so you can be sure to get the right one:
https://www.harborfreight.com/small-modular-toolbox-58510.html
 

Including the description of the toolbox so people can actually find this thing when keyword searching:

The BAUER™ Small Modular Toolbox is ideal for organizing tools. Made from high-impact resin, this toolbox has 6 removable organizer trays with lids. Strong metal latches keep tools secure during transport. With a 25 lb. weight capacity this toolbox is fully compatible the BAUER™ Modular Storage System. The organizer is IP65 rated for dust and water resistance. Customize your storage for quick access to essential tools.

• 6 removable organizer trays with lids
• 25 lb. weight capacity
• Strong metal latches keep tools secure during transport
• IP65 rated dust and water resistant
• Rugged construction, built for durability
• The BAUER™ Modular Storage System stacks and locks together for full customization to fit any tool storage needs