Best isolation platform for a turntable with suspension?

DYI a wall mount shelf for your table could buy one too but easy enough to make a shelf.  That will help greatly. Most likely nothing else required after that.   

I use a wall mounted rack with a Mapleshade 2" maple platform for my Avid TT in my second story wood floor room. This works well for me.

I did a whole lot of research into this, and tried a lot of different things, all during which time my Basis table was used on the floor. Can't recommend the floor as being very convenient. It is however pretty much the gold standard for turntables. Very hard to build a rack as good as being on the floor.  

Hard, but not impossible. This does the trick. https://systems.audiogon.com/systems/8367
Like slaw recommends its heavily BDR. Like Mike mentioned its mass loaded, uses sand, and is incredibly cost effective. The whole rack including granite can be built with a minimum of tools for only about $300.  

The whole rack is over 700 lbs. But it is modular design. You could build just the top shelf with or without the granite and put it on top of your existing rack. Then add the rest later. Or not. 

If you do build the whole thing, or even part, I am sure you will find it can be done better. But for thousands, not hundreds. 

I have my Basis TT on top of a large BDR shelf.

My reference TT without suspension rests FLAT on top of a CMS shelf, it works great.

Good luck,
Jose

I use  marble  I got from local counter top guy...I love it.