I am in the process of building a plinth. Originally I was going to use Jatoba - a very dense wood that takes a very smooth finish. However I came across a piece of walnut with good grain and a nice color (a bit less red than typical). Walnut is typical of the mid-century woods.
I'm going with a simple design, but it won't have the lip that John demonstrated on the original S-C plinths. Inside the plinth will be a 5/4 Oak liner. This will add weight a structural stiffness. I don't believe a heavy, solid core plinth will be advantageous for this table, given that it is suspended. the plinth will probably be about 10-15 pounds when complete.
Here is the rough version:
View attachment 1802927
Notice the offset liner at the front left corner is needed to clear motor and its suspension.
The top plate will sit in a slight rabbet so it is approximately flush. The rabbet will be stained dark to make it disappear.
View attachment 1802928
The edges will be treated with a round-over bit in a router. It's hard to see in the photo, but there is an offset base, and it will ultimately have some small feet. At this point, I don't anticipate any other treatments or joinery. Weather permitting, I'll start the finishing process tomorrow. I'm going to use a couple coats of oil/poly by Sam Maloof followed by a couple coats of his wax/poly.