This is a simple that can just plug in to a 300 amp Anderson connector, which are standard on forklifts and other similar electric equipment; at least in the US. All you need to assemble this is a panel mount volt meter, two paper clips, and glue.
Instructions:
Volt meter link: https://www.amazon.com/dp/B0C5998JPZ?ref=ppx_yo2ov_dt_b_product_details&th=1
The author marked this model as their own original creation.