Okay, I want to be clear that I’m not at all an expert on this, I’m just working with some high school physics from 30 years ago. So my ideas are just ideas and should be investigated before any major investment is made.
If taking the printer out of the enclosure reduced the ringing, then the goal is to reduced the movement of the enclosure. That movement could be coming from several places.
The castors are certainly one obvious place. Castors revolve in 2 axes, and cheap or old castors will usually have a lot of play around each axle. Taking the castors off would eliminate all of that. But I don’t know how important it is that you be able to move the enclosure around.
If taking the castors completely off isn’t ideal, try immobilising them instead. remember you’re not just trying to freeze the motion off the wheel, but also the pivot.
The frame of the enclosure might also be flexing and vibrating, inducing more motion that’s causing ringing. Removing the castors might help with that, as the frame would presumably be sitting on the floor and thus have less space to flex in.
Replacing the body panels of the enclosure is probably cost-prohibitive, so we’ll set that to one side.
Having additional bracing might help. I don’t know, I’m just guessing.
Finally, the enclosure is moving because of energy transfer. Which means that either the printer is managing to get that cement block moving and then the block is transferring energy into the enclosure, or else the cement block isn’t absorbing enough energy in the first place. (I’m sure I’m screwing up the engineering here.) Anyway, the next experiment I’d consider is changing that mass. Does the ringing get worse if you take out the cement block? If it does, does it improve if you add another block, either under the printer itself or just as ballast in the enclosure? What happens if you take out the foam? Things like that.
And, of course, all of the energy is originating in the bed moving back and forth, so of course slowing down the Y-axis speeds ought to reduce the ringing. No, I don’t have any idea if one can change just Y-axis speeds without changing X-axis, nor do I know if you’d want to. But, basically, for anything where final appearance matters, you might consider just slowing the printer down.
At the end of the day, science is trial and error, plus taking notes. Time to do some science!