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 Milwaukee M18 or DeWalt 18V XR 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.
Snap In Place - Your batteries will clip in place just like on your tools, it's a modular design so brands can be swapped out if you change tool platforms later on.
M18 Batteries - Designed to fit standard M18 Redlithium batteries ranging from 1.5AH to 5.0AH. More details provided below.
DeWalt Batteries - Designed to fit 18V XR batteries, up to 5AH. Flexvolt have not been tested, I assume they're too big.
Makita Batteries - Coming soon!
AEG Batteries - Coming soon!

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Battery Compatibility:

It specifically fits the standard M18 & DeWalt batteries ranging from 2ah to 5ah.
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.

Supported Batteries - Links:

M18 Redlithium 2ah - https://www.milwaukeetool.com.au/power-tools/batteries-and-chargers/m18-batteries/M18B2.html
M18 Redlithium 3ah - https://www.milwaukeetool.com.au/power-tools/batteries-and-chargers/m18-batteries/M18HB3.html
M18 Redlithium 4ah - https://www.milwaukeetool.com.au/power-tools/batteries-and-chargers/m18-batteries/M18B4.html
M18 Redlithium 5ah - https://www.milwaukeetool.com.au/power-tools/batteries-and-chargers/m18-batteries/M18CB5.html
DeWalt XR 2ah - https://www.dewalt.com.au/product/dcb184-xe/18v-xr-5ah-battery
DeWalt XR 4ah - https://www.dewalt.com.au/product/dcb184-xe/18v-xr-5ah-battery
DeWalt XR 5ah - https://www.dewalt.com.au/product/dcb184-xe/18v-xr-5ah-battery

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