This is a secondary battery tray for Tamiya WT01 chassis. It allows you to use physically larger battery pack. You simply hot glue it to the chassis.
This is version 1:
upgradeparts-tami-wt01-secondary-batt-tray
This is version 2 (longer):
upgradeparts-tami-wt01-secondary-batt-tray-long
These videos show a Wild Dagger equipped with this mount and a 3S: