Best type of metal for turntable platform?

You don't need CNC, unless you yourself make it a requirement. What you need to eliminate footfall problems is a two stage approach. First stage, mass eliminates large amplitude low frequency footfall type vibrations. Second stage, springs isolate the turntable from the smaller amplitude higher frequencies that get past the mass. 

With a superior product like Townshend Pods your table will probably be fine even with your existing rack. People who have PM'd me about problems like this tried Pods and they work great in situations much like yours. They work even better in situations like mine, which is why it is still a good idea for you to build a sturdy low profile rack with a massive sand bed. As described above. 

You really should as a general principle ask the question you want answered. You came here asking about metal when really it is about footfall problems with your turntable. At least that is what it seems to be. Until it changes yet again....?

Its not the metal. There are no metals that are good for vibration control. The reason you see all these metal racks is the same reason for all the MDF speakers- these materials are inexpensive and easily machined.

The place where you get the most benefit isn’t the material, it is the design. Straight lines and 90 degree corners, these are used for the same reason as metal and MDF: ease of manufacture. For a one-off project you will get much better results using curves. No straight lines. No parallel shapes. Look at a tuning fork- straight parallel aluminum. Rings and rings all day long. That what you want? No. So no straight lines.

Your best bet will be to incorporate springs into the design. Springs are by far superior to anything you can do with cones, spikes, etc. The trick with springs is to determine the right amount of spring stiffness for the component load. I would get Nobsound springs, as they are small and extremely cost effective, making it easy to add or subtract springs according to whatever load you wind up with.

They are only about 1" high, about 1/2" under load. Then design your shelf. It can be MDF or a sand bed. Using CNC you can machine a sand bed only about 1" deep (thick) to fit within the rack frame so that it looks almost like one piece. The frame itself should then be gently curved across the front, with a tighter radius at the corners. The front, back and side rails should be flat on top but curve like a bridge arc on the bottom.

If you will be using Townshend Pods under your component then you can eliminate the Nobsound springs, incorporate the sand bed into the rack, and make a simple 1/4" plate of aluminum to cover the sand bed. Powder coated this will look rad and work like you won’t believe.

By the way, I have no idea why you're talking such tiny dimensions. Unless your turntable will be on the floor. Which works great, but incredibly inconvenient. If you want a turntable rack that is at normal comfortable height, relatively easy to build and incredibly cost effective, let me know I will tell you how to build this one. https://systems.audiogon.com/systems/8367