It specifically fits the Milwaukee 48228422 PACKOUT Compact Tool Box:
https://www.milwaukeetool.com.au/storage/packout/tool-boxes/48228422.html

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Features:

Easy Storage - Provides a compact and protected storage solution for your assortment of M18 Batteries! The insert simply drops in from the top and sits securely on the provided lip inside. It functions in the exact same way as the stock drop in tray that comes with the box.
Handles - Recessed handles are provided at each end of the insert to help you with lifting out the insert and access gear in the bottom of the box. Two additional handles are provided along the sides for extra options as well as a lip that runs the entire side for extra grip.
Snap In Place - Your batteries will clip in place just like on your M18 tools! You can turn the entire insert upside down and the batteries won't move at all. To insert, push the battery into the slot as you would with your tools. To remove, press the side buttons to release just like with your tools and lift up.
Ergonomics - It was a tight fit but Jonah was able to fit slots for EIGHT M18 Batteries! To aid in the ergonomics, the endmost batteries in the corner have recessed channels to allow for better accessibility of fingers to release the batteries. This way you can maximise your use of space rather than sacrificing two slots and only fitting 6 batteries.
M18 Batteries - Designed to fit standard M18 Redlithium batteries ranging from 1.5AH to 5.0AH. More details provided below.

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Battery Compatibility:

It specifically fits the standard M18 Redlithium batteries ranging from 1.5AH to 5.0AH. The design won't support sizes larger than 5.0 purely due to the huge increase in weight those batteries would add. As the design supports being lifted out to access gear stored underneath, anything heavier than eight 5.0 batteries may present an issue for the structural integrity of the print.

High Output and Forge Note - I have not been able to test the new M18 Forge Batteries or M18 High Output Batteries. My research indicates these are larger/wider batteries thus wouldn't fit in this design due to the bigger footprint.

Supported Batteries - Australian Links (our batteries have different label graphics):

Redlithium 2.0AH - https://www.milwaukeetool.com.au/power-tools/batteries-and-chargers/m18-batteries/M18B2.html
Redlithium 3.0AH - https://www.milwaukeetool.com.au/power-tools/batteries-and-chargers/m18-batteries/M18HB3.html
Redlithium 4.0AH - https://www.milwaukeetool.com.au/power-tools/batteries-and-chargers/m18-batteries/M18B4.html
Redlithium 5.0AH - https://www.milwaukeetool.com.au/power-tools/batteries-and-chargers/m18-batteries/M18CB5.html

Supported Batteries - USA Links:

Redlithium 1.5AH - https://www.milwaukeetool.com/Products/48-11-1815
Redlithium 2.0AH - https://www.milwaukeetool.com/Products/48-11-1820
Redlithium 3.0AH XC - https://www.milwaukeetool.com/Products/Batteries-and-Chargers/M18-Batteries-and-Chargers/48-11-1828
Redlithium 4.0AH XC - https://www.milwaukeetool.com/Products/48-11-1840
Redlithium 5.0AH XC - https://www.milwaukeetool.com/Products/48-11-1850