60 Hz noise shielding suggestions?

Discovered a 60 Hz hum getting picked up by a cassette deck (Denon DRM-710). The noise is being generated by a new TASCAM DA-3000. My system rack only allows me to stack the Denon on top the TASCAM and there’s only about 3/4" of available height left. The noise is not picked up by either of my turntables, one with a DV XX2 Mk II and the other with an ATC 440 directly beneath the TASCAM, so either attenuates with distance or the Denon is especially sensitive to it. Everything is electrically isolated, and the hum is audible as soon as the tape heads are energized. Turn the TASCAM off and it’s gone.

I only have two solutions: Remove the Denon from the rack or come up with some kind of shielding material to put between the TASCAM and the Denon. The TASCAM’s presence in the rack is required. Moving the Denon is middling to very irritating on the inconvenience and WAF scales, so a shielding medium would be most preferable. Does anyone have a material suggestion that might work? I’ll look forward to your input!