Thanks for your input, Ron. A good friend of mine took detailed notes at our club Ellsworth demo last year and shared them with me yesterday. The recommended rod was 3/16" galvanized (or zinc coated) that is sold in three foot lengths at hardware stores. As you said, the rod is bent into a circle of sorts. I would add only that one tip is bent to aim at the other at a right angle rather than having to the two tips meet. This is not a precision instrument but rather one that can give you a general impression about the consistent wall thickness of your hollow form. To use, adjust the gap to 1/4-inch or larger, run the tip along the inside edge of the vessel and watch the gap along the outside shape to detect uneveness. You can easily change the shape of this jig to conform to the contours of the piece you're working on.
As you said, Ron...pretty low tech but useful.