US source of Panzerholtz?

Right Teo, you need diamond tooling for this. Rich, far cheaper, easier to get and deal with is solid surface material. I like quartz products best. You can go to installers and get cut offs cheap. Laminate it with a plate of aluminum in between and you'll have a wonderful plinth.  

bdp24, Corian is very difficult to get if you are not a licensed installer but there are other brands available that are essentially the same stuff. Corian will actually stain over time and is not as hard as Quartz solid surface material which will never stain. I made my sub woofers out of Corian layered with glass microspheres in epoxy (System 1). Layered with MDF as Phoenix did should work well also. If you edge the MDF with a hardwood like walnut you can get nice cosmetic results. Quartz is harder and heavier than Panzerholtz but more brittle. You can not use only a hardwood for the middle layer as it will expand and contract with humidity and in time the whole assembly will fall apart. I have also made dipole speakers this way with excellent results. Quartz really requires diamond tooling, Corion will cut and shape with carbide.

Corian does not have flakes of aluminum in it. It is alumina trihydrate and an acrylic polymer. The different manufacturers of solid surface material use different polymers. Alumina trihydrate is a byproduct of bauxite as is aluminum ore. There are now about 20 different manufacturers of solid surface material now including the quartz versions. All of it is very heavy and stiff. Laminated to MDF it makes a marvelous plinth or baffle board. You just have to have the tools and knowledge to work with it. It is also extremely messy. The dust sticks to everything. You have to vacuum your ceilings after a project. You have to use special adhesives that require special guns which are different for any given manufacturer and the adhesives are color specific. For the usual home crafts person it is just plain hard to deal with it all.

Rix, 4 mm steel plate? For a speaker enclosure? Do you have any idea of how much that would weight? How big is the speaker? Another issue is that steel rings, aluminum does not. My buddy's Magico S7s are aluminum and they weight 300lb each!