From the designer:
This is a custom insert that I have designed to fit your Milwaukee Packout to store your M12 Detail Sander and accompanying kit. It specifically fits the Milwaukee 48228435 PACKOUT Compact Organiser.
The specific M12 Detail Sander that this is designed for is model M12FDSS0, link is provided here: https://www.milwaukeetool.com.au/power-tools/carpentry/sanders/M12FDSS0.html