Best type of metal for turntable platform?

Antinn gave you good advice. Constrained layer damping is an important element in most plinth designs - it’s often absent and that’s always obvious.

If you want an easy solution, I use Delignit panzerholz B25, which costs about $1000 a sheet. In my plinth, constrained layer is partly in the plies of beech wood in the panzerholz, and partly from a top layer of carbon composite.

Rigidifying your floor is even more important. My floor is concrete - it’s great - and as others have suggested, try to make your floor as much like concrete as possible.

Good luck!

If you're dead set on using an aluminum plate for a plinth, then the alloy makes a big difference. Alloy 2024 is recommended by some who have experimented, and the temper does not seem to matter much; all tempers seem to have about the same loss modulus, which is a lot better than alloy 6061.

I need some feet under a TT plinth (which just nothing fancy, just about 6" stack of Baltic Birch ply) and was thinking of trying the Townsend Pods. Would I divide my roughly 100lb load by 4 and use 4x pods that accommodate roughly 25lb each?

Solid gold, of course. 

To remove the Floor Bounce issue and Vibration Transmission to the 
TT.
A TT Wall Shelf can be used.
It will be at it most effective if a Wall is Bricks and Mortar Structure, if  other structural materials are used, it will be best to seek out the ideal fastening to secure the TT Shelf.

When Looking a a fastening for a Wall Shelf, Do Not ! just delect a fastening as you are familiar with it.
Seek out the ones that has the best Kg Loading for the substrate to be fastened to.
I found mine on Youtube and when fastened into Cinderblock, they are 250Kg per fastening.
There is nothing like such reassurances.  

A DIY Sub Plinth can be made that can control various effects from Vibration.
A low profile 'Side Wall'  1 1/4" (30mm) Tray can be made that is to contain a approx 5/8" (15mm) Depth of Kiln Dried Sand.
This box can be sat on Sorbothane or Similar Footers.

An additional Box will be made, that fits snuggly into the Kiln Dried Sand Tray, a 1/8" (3mm) clearance all round between each boxes 'Side Walls'
should be ample space.

A base can be cut for this Box, that is 1/4" (6mm) smaller that the inside dimension of the Trays 'Side Wall'.

The following is a Crude Version of a early Townshend Design.
It works on par with any type of design using the concept.
 
This Box will be a Housing for a Inflatable Inner Tube, a Side Wall will have a Vertical Slot cut in,  to allow the Valve to exit the internal space.
The Slot allows the Valve to elevate freely when inflating.

The Height of the Side Wall will need to be determined by how much inflation is chosen for the Inner Tube.
When the ideal inflation is discovered  the Inner Tube can be placed on the Kiln Dried Sand, a Dimension can be taken from the base of the of the box that will be approx 10mm above the Kiln Dried Sand
(The inner Tube should be close to 10mm above the Side Wall when inflated, do not make a excessive dimension, as the 10mm will allow the assembly to find a resting place in a controlled manner if a inflation is decreased over time ).
Place the Box with inflated Inner Tube onto the Sand, with the Inner Tube seated on the Sand.

Place the TT on the Box/Sub Plinth and check the TT for Level.
Footers under the Plinth will assist with achieving the levelling that is required, as well as offer a further Vibration Control.

Footers are also going to attenuate the sound, so finding a ideal one is a
personal experience and down to ones preference.
My Footer of choice in direct contact with a TT Plinth is 
 Solid Tech 'Feet of Silence', these were put into Service after using
AT-616.
As said the choice becomes personal.         

Note: The Box with the Inner Tube can be seated in reverse with the Boxes Sub Plinth being seated on to the Sand.

This will allow for 'different material Sub Plinths' to be seated onto the Inner Tube, this is another way to fine tune attenuation to suit a personal preference.

This assembly can be achieved at a reasonable cost and a allocation of ones time, it will offer a vast improvement over a conventional Support/Rack that is in direct contact with floor.

The cosmetics for the finishing will be a little more to be added to the parts used, if the raw honesty is not desirable. 

Apologies for the Long Mails, I'm living in a Lock Down, so Coffees and Keyboard are part of the Day Filler.