It’s not copper around the transformer it’s MU metal that is wrapped in a red plastic covering. I did use a piece of copper foil; but only to ground the metal air vent I added to the metal of the lid.
Yes, I know. That’s why I said “if”. 
Hmm. It would be more effective if a circle were cut slightly bigger than the diameter of the toroid, placed 5-6mm above the transformer. If you have any left, give it a try.
In purist terms (IMO), garbage. If it is mu-metal, putting holes through it essentially ruins its effectiveness. If it isn’t mu-metal, then it is entirely useless - if its intent is to control magnetic field leakage.
It looks like the newest version of the transformers has the Flux band made of (probably) steel. I just tested the Triad Magnetics VPM12-2080. It has it, too. Not MuMetal, but some sort of a help.
There are so many weaknesses in this DAC we have to address. The newest version of the transformers is not one of those. Although, there’s a chance you could find something better, but it’s a good quality transformer.
In that regard, Triad Magnetics is superior to the newest version (Amgis) because it has a shielding between the primaries and the secondaries.
I wasn’t actively looking for a replacement for the power transformer. I found that we could use that specific Triad Magnetics VPM12-2080 transformer by accident. I suggested to use it not because it sounds better (I don’t know that for sure, but I trust Sparkos who said they sound good), but to eliminate the heat issue that was brought up to everyone’s attention recently again.
A cap MuMetal shield. Even if any holes make them less effective, they still work. Do we need them here?
For the toroidal transformers, we don’t need that, the Magnetic field is already too low, and in the newest stock transformers, there is a flux band already implemented. I don’t know about the newer model, though. (There are three different transformers in PerfectWave and DirectStream DACs)
In my DAC, I added a copper shield between the two power supplies and the analog board, the processor board, and the Bridge II card.
On the other hand, the R-core transformers will benefit from magnetic shielding. They usually have a copper flux band, which is good. But they have worse magnetic field leakage than the toroidal transformers. That’s why I put a MuMetal cap on my R-core transformer for my PSU for the analog board.