Below you have the prototype schematic. Remember that in the video I've used a BC547 NPN at the gate of the MOSFET but is better to use a PNP and have always a pulldown at the gate of the MOSFET for safety. The current sensor is the model of 5A but you could use any oter but remember to change the multiplier in the code. As for the relay, you could buy directly a module and don't use the small NPN to activate it. For the power lines I recommend you to buy thick copper wire. Get the small buck converter and fix it to 5V and glue the potentiometer. Please check the part list above and the vvalues on the schematic below.