I can only access part of it. But, have a fairly large surface area to work with (about a 3" diameter slice).
How can I get a *reasonably* good estimate of its actual radius? I thought, perhaps, a "precisely" cut circular opening that could sit on the surface and then use that (knowing its thickness) to measure to the "peak" of the surface protruding within (?) E.g., a dial indicator riding on that material in which the opening is cut...
The rest being math.