Good Old Chipboard

"Besides that I would think company's like VPI, Clearaudio and others with R&D departments have done just that."

On average, I'm a pretty stupid person.  But sometimes people just obey conventions and presumptions without observing all the options. I know what I know, and some of what I know I know.

Well you know what they say when you assume things. To quote Mark Twain, " it's not what you don't know that will hurt you, it's what you know for sure that isn't ". Just saying.

I've built the same turntable using different materials for the plinth. Also made cones, footers, shelves and racks using different materials. After a while you gain enough experience you realize you can get a pretty good idea what something will sound like simply by tapping on it. The sound it makes when you hit it is the sound it will impart to whatever you build out of it.

MDF is so widely used not because it has such fantastic vibration control characteristics. Its used because of its unique combination of features. Mainly, its affordable, available, uniform, easily and precisely machineable, and has reasonably good sonic qualities. Build the same thing out of solid pine, oak, teak, or any species wood, and compare to MDF it will be apparent each wood has its own character and all of them together have much more character than MDF, which is comparatively neutral and benign.

Chipboard is something I have never tried simply because it is so obviously a worse material than just about anything else. Cardboard, maybe, would be worse. Soggy MDF. Sheet rock. Whatever material you could find that is even less uniform, with more voids, less stiffness, and more impossible to shape with any degree of accuracy.

But hey, go ahead and try. That is after all how most people learn.

What I really am curious about though is where in the world you ever heard of "good-quality good old fine-grained chipboard"?

That to me is such an oxymoron it almost makes me think you might just be a troll?

If you were walking around in the 1950s you would have seen it around.  You seem to have a chip on your shoulder about chipboard.
And....MDF IS cardboard.

@mrearl, I thought it was only me that felt this way!

"MDF is supposed to have very little inherent sound, but the inherent sound that it does have is yucky."

MDF is cheap, easy to work with, easy on tools and easy to prepare for lamination. But that’s about all that can be said for it unless maybe you know exactly where (and how) to damp those life killing tendencies.

The use of chipboard, especially for loudspeaker cabinets declined rapidly once MDF became available. Some have resisted the desire to go down the MDF route on sonic grounds, but it always comes with increased cost.

Chipboard may well sound better than MDF for budget/entry level turntable plinths, but without lamination it would be a hard sell, as you say.