Pretty much any metal that deals with clothes (which is what the main purpose of these cubes are for) is plastic coated to make sure there is no chance of it rusting onto the clothes. But if you are worried about it, just cut some plywood to slide in and mount everything to it.
I'd advise against plywood. In the 1/10000 chance that something sparks, shorts, or overheats critically you want as little flammable fuel possible nearby. Plus, wood does actually conduct electricity. It also retains moisture. Some plywood can also emit toxic fumes when heated up.
I know that all sounds paranoid. So take it as you will.
That being said, in more than 20 years of professional experience working with computing technology, I've only seen one machine come close to igniting. A NeXT cube optical drive in my lab (back in the 1990s) got hot enough to start smoldering. Thankfully, I smelled the smoke in time. I'm 99% sure it was the dust that had built up in the drive over the years.
I suppose you could also use some adhesive spray on glue and aluminum foil to create "fire resistant" plywood slabs too. But you'd run into the fume issue there too I suspect.