Best type of metal for turntable platform?

These are great pointers!

Among metals, aluminum is one of the best to dampen resonances.

Are you having footfall problems?

what do you mean by footfall?

If I were to machine a plinth, I’d choose Corian, rather than any metal.      Even if it meant stacking and gluing sheets; to obtain an adequate thickness/depth.      I’ve used the stuff, in a number of installs/applications, for resonance control.      If your tools/CNC router bits are sharp; it machines nicely and can be polished to a beautiful finish.      It’s resonance damping characteristics are much better than any metal.     A thread regarding plinth materials: https://www.diyaudio.com/forums/analogue-source/312473-diy-cld-plinth-design-measured-approach.html