Portable PSU 1.5v to 18v (3A)

Two 4mm holes need to be drilled into the battery adaptor. The bracket is then secured with M4x12 screws and standard M4 nuts. M3 inserts are melted into the bracket. The buck converter is then attached using M3x8 screws.

Makita battery adaptor:

Battery Adapter Power Wheels Adaptor for Makita 18V Also for Bosch 18V Battery BL1820 BL1860 BL1850 Dock Power Connector Holder with 14 Gauge Wire for Power Tool Batteries Mounting Frames: Amazon.co.uk: DIY & Tools

Buck converter: (3A Max)

4 Pieces Adjustable LM2596S DC-DC Buck Converter Reduced Voltage Regulator Power Module 36V 24V 12V to 5V 2A Voltage Stabilizer with Digital Voltmeter Display : Amazon.co.uk: Business, Industry & Science

On off switch (SPDT - SPST would also work):

The hole in the bracket is 6.4mm OD

1MS1T6B11M1QE Multicomp Pro, Toggle Switch, On-On, SPDT | Farnell UK