Best type of metal for turntable platform?

As in most of life, there is no best anything. 

Of course not. But I think SME tables take care of everything (no shelves, pods/platforms, etc.,) from what I've heard. Not inexpensive though...Sound like an end game for the whole source discussion.

Given that you can have your own platter machined you should do the following:  It should have a hollow space underneath.  Think Technics SL1200 platter.  It has a flat top (of course), and then at the edges it has a lip (with all those dots).  Underneath is a rubber underlay.  Do something similar but rather than a rubber underlay you will pour epoxy (g-flex is good).  This will damp the platter.  This is important if you are having a metal platter, otherwise it will ring like a bell.  The constrained layer damping suggestions are also good.

As regards footfall I have two suggestions.  First add some mass.  For example I use a one inch slab of granite on my shelf.  Second get some aftermarket footers.  There are maglev footers on ebay,  Even better are Isonoe footers - they are in effect mini trampolines.  I can jump up and down next to my TT without adverse effect.

andrei_nz

Thanks for the info. Everyone on here has been very helpful. Do you know where maybe I cant find an exact schematic? I know that's asking for too much. He's the type that just needs the measurements and/or plans. 

Its puzzling to me how mini trampoline footers like the Isonoe can can be good for footfall.. I am intrigued though and might try them.  Once it warms up here, from influence of these replays, Im going to get under my floor and add supports to the floor. BUT! If I can find a way out of it.. Ill try it (without breaking the bank).

The floor supports will do the most if you can pull that off. I couldn't so had to do the wall shelf/platform I mentioned above. Your sound will improve dramatically when you remove all the vibrations!