These rings were designed in TinkerCad. TinkerCad is an easy-to-use 3d design program which is great for beginners, or for anyone who wants to quickly model simple shapes.
If you don't have access to a 3d printer, you can order them from Shapeways here:
If you are printing your own, download the files below. I've included all three sizes in the .zip file, so select and print the one that fits you best.
I printed in clear PLA, with a raft and supports. I had the most success when I printed the widget lying down on its side.
My clear PLA didn't really come out clear -- it looks really white, and shows up more than I'd like. A flesh-colored filament would probably look better if your character is bare-handed. If your character wears gloves, print in a color to match the gloves.
If none of these sizes is quite right for you, it's easy to edit the files in TinkerCad to get a perfect fit.
Do not shortcut and simply resize the whole .stl file before printing -- the top end is sized to screw into the Circuit Playground case and if you resize the whole thing, it will no longer screw in tightly.
Select the size that's closest to the one you want. Click the "ungroup" button in the upper right corner.
Click on the rings to select them. Click the white box in the corner and you'll see measurements appear. Hold the shift key while you drag with your mouse to resize without changing the aspect ratio.
This looks better, but now we're mis-aligned. This is easy to fix. Drag a box around the rings and the stem to select them both. Click the "align" tool in the upper right, then click the dot in the bottom center to align the rings with the stem.
Once you're happy with sizing and alignment, select the rings and stem and click the "group" button to attach them back together.
Select your resized widget and click the "export" button. Choose "selected shape" and "stl" to download your updated file for printing.
If you want a longer or shorter stem, you can use the same method. Just be careful not to resize the screw tip.