So I ordered some mu-metal sheet and applied it today. For each of my two noisy devices one layer directly over the lower side of the transformer and one on the housing beneath (see attached images).
It definitely helps. With just the device closer to the scope turned on it’s just about noticeable, with only the farther one turned on not at all (when I previously had the shielding only applied to the closer one, the farther one made more noise than the closer one). Interestingly, when I turn both on, the noise is most noticeable (the farther one here seems to increase the field, despite not having an effect when run on its own). But even then it’s still acceptable and probably less than only the farther one before.
Maybe it would help to also add a piece of mu-metal to the top of the scope housing? But honestly, with the cost of this stuff I guess I’m happy with it right now (I was lucky and got a piece from an ebay auction comparatively cheap).
Edit: But if you attempt to do something like this, too, be careful with the edges. I cut my sheet with a paper cutter, that worked out great, but it left an edge sharp enough to cut myself in the finger

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