Best type of metal for turntable platform?

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. 
  

Hello,

I remember audiophile discuss we had during  the vinyl  " golden age" from middle of seventies to eighties"
We had discovered the " sound" of the turntable highlighted by the audio litterature ( Ivor tieffenbrun Linn sondek)

In general two approach have been acknoledged for the structure design

The ratio of mass
- plinth -platters-tonearm-cartrige tracktability( high or low)
A)
High mass plinth 
High mass platter
High mass tonearm
Low cartridge tracktability

In this configuration the transmission of the energy of vibration is balanced
B)

Low mass plinth
Low mass suspended (or not) platters
Low mass tonearm
Low tracktability cartrige

In that case, the ratio of mass structure between  the elements is also balanced

So, what ever the platter structure is the " bell"  effect must be be reduced keeping in mind that each component have specific " sound" never neutral..

I own two vintage turntables  whitch are complying  perfectly  these rules as example
- Thorens TD 124 II 
( injected aluminium & cast iron)
- sme 3012/2 & DL103)

-Walker Cj 55
Wood and suspended platter in tufnol ( the most neutral backelit phenolic component to be used in aircraft for years)
Colin walker was engineer who used to work for castle engineering in the seventies and launched the linn sondek and Ariston RD11s in uk, Usa and australia inthe seventies...

Sorry fpr this backward looking story !
It is just my experience to be shared...

Raymond

+1 +1 +1 noromance

You sure got it correct.
Concrete is the answer to vibration and indeed marble and stone, and plenty of it.

OP has a suspended floor above crawl space and bare earth below. Great start.

Rolls Royce solution:  Take up the floor, dig out a bit if required, lay new DPC, in-fill with concrete screed, lower floor level to get more ceiling height if desired.  According to taste, re-lay boards except where equipment and speakers are to stand.  Or lay carpet.
OK there is a cost, but it will be worth it.

Use marble or stone blockwork for equipment supports, standing on spikes (PLEASE NOT SPRINGS - BOINNGGGGG...GGG...GGG!).  All you equipment will be rock steady, no vibration.  It is mass-loaded to Planet Earth.  Infinitely mass-loaded as I have said.  OK, not quite infinite but the mass of the earth is quite large.

This is absolutely excellent for turntables and speakers, good for CD players and amps benefit too.  There is no ground-borne vibration (except in California) and the mass damps away all air-borne waves.

I have implemented this construction in my sound engineered basement listening room.  Improvement to SQ is much bigger than many component changes.

Target wall mount TT stand fastened to a load bearing wall.  $150 and you are done.