The Topping P50 has a maximum output of 25 watts, if the barrel plug and both USB ports are at 100% load simultaneously. That is so little power, your wiring doesn't need to be pure copper, and the thickness is almost irrelevant. As a rule of thumb, I round up to the nearest full amp when selecting wire gauge. By that logic, anything around 22AWG should suffice.I just need a cable built from pure copper, with some shielding (?), possibly not thin as a hair.
For reference, 22AWG is 0.327mm cross section.
Shielding makes no sense on AC mains power. The power itself is already "dirty", 50% of the job performed by any power supply is filtering out that noise, and requires no special cables or high-end gear to accomplish. If we are being objectively honest here, you could power the Topping P50 with 3 rusty coat hangers and not make the slightest difference to the sound quality.