Uninsulated bare copper wire for turntable ground OK?

Sure. But is life that hard that you cannot procure a yard of wire?!

Perfectly OK to use bare wire for ground. Thick gauge best. But I don’t see any reason why it’s better than insulated wire of the same gauge and composition. Nor should the dielectric make any difference.  If I have some heavy gauge 3-wire Romex cable lying around, I like to strip out the bare ground wire and use it as ground wire in components.  You can get 12- or 14-gauge Romex at any good hardware store.

I put a 20 gauge OCC banana extension on mine. It is perfect. Tell me I’m wrong.

Yep.  It's fine.

You need insulated grounds for safety grounds (which this is not), but if it contacts any other metal which could have a potential it could put a lot of noise into your phono. I'd suggest you pick up any cheap hook up wire from Parts Express or the like.

Thanks all!.  My maintenance personal had bare copper wire at my business so it would be convenient.  I just wanted to avoid indoor stores (hardware) as I am in Los Angeles where COVID is raging.  Turns out, I needed to buy a door knob for my wife's closet today so I got insulated primary copper wire (solid core, 14g) and a set of spade terminators.