So even if there were some leakage the amount of voltage to start with is so low that I doubt there would be an appreciable risk.
So, if I understood correctly, through a power supply it is not possible that the AC current arrives inside the case?
The best way to reduce the risk of any sort of leakage current would be to use a metal case and make sure the case is grounded. The location of the power supply (internal or external) shouldn't matter so long as it is properly grounded. And as a general rule, don't cheap out on the power supply.
There is a good centralized and certified grounding system in the building where I live
The one exception you might want to be careful about are modules involving tubes. Some tubes involve quite high voltages, sometimes a few hundred volts, and that is obviously a greater risk.
For sure, tubes are more dangerous, but I don't plan to buy modules with tubes insides
Thanks Replicant!