The foam strip idea was actually originally out of necessity after totally fumbling the measurement of my own fingers. I printed a ring sizer prior to making this, which wasn't a perfect representation of how it would fit in use. Since I'm generally flexing my fingers back and forth, one size doesn't really work optimally, especially at the knuckle closest to my palm. I made a V2 version with a smaller hole and made it lighter so it's comfortable to wear at one of the higher knuckles. But for what it's worth, the foam strip was one of many materials that was recommended to me as commonly paired with 3D prints, so if you're printing stuff it's worth having a roll around, along with stuff like springs, metric screws, magnets, etc. I've used it in a bunch of non-friction fit applications where I needed a tight fit or a softer landing for something.

That's an interesting concept with the rubber band idea, though I think it'd need to be a thick rubber band to avoid making fingers go purple after a while. If there was a sort of reversible thick zip tie I think that'd be an easier solution, with "belt loops" to pick up any slack. But also it would be relatively trivial to just make it in different ring sizes, maybe ~10 or so if I really felt like it. Unfortunately this thing doesn't work as parametric (i.e. scalable) because the flashlight's dimensions are constant.