So here is a CAD drawing I did in OpenSCAD of a concept for a ring with a screw-on top. the top has a hole in it for an LED. There is a side hole in the ring for a mini-usb connector. That needs to be shaped a little more but good enough for now. Note the top has a cavity to increase the payload space. To think about the size of this, the ring will screw onto a 2-litre pop bottle like a bottle top. so vertically large, but we can overcome that later when we physically prototype.

This is a concept drawing, and the first one I have done in OpenSCAD. As a result it is untested and basically a proof of concept. Putting it here as it may be useful to others to have the basic layout done. should save you a couple of minutes at least...

I'm thinking you make a small electronics package, based on small arduinos, and a coin cell on the bottom. glue it to the top in the recess, then screw the thing into place You can groove the top so you can use a coin to remove it later to change the coin cell battery it would run off of, otherwise it sits flush.

Need to figure out mechanically how to get the wiring in it for the connector so it lines up right, but we can solve for that.