Here's a part of a cabinet in my wardrobe where my printer lives. It's a bit noisy with all those hard surfaces so I am just about to put up some foam padding on all 5 sides.
Is that stupidly dangerous?
You can see I have a smoke alarm there, but it won't stop a fire on its own.
Edit: the cabinet has no door, it's always open like in the photo, but the wardrobe door is generally closed. The room has some ventilation so smells do go away.
You could use a lot other material in that case. The concrete itself is a non-sequitur, your isolating the base with another base with rubber between both. Concrete, wood, plastic. Anything at that point between the two rubber pieces.
Massive, won't melt, won't catch fire. Ceramic tiles would work just as well imho, though a tad lighter (which might actually be good, given the thickness of that shelf)