After playing around for some time with some Dyneema fishing line and a micro cord lock to secure my hearing aids when I am in the bush or canoeing (which by the way worked well) I am delighted to learn there is a commercial alternative which whilst not so secure, is a lot simpler. These devices are called: Hearing Aid Oto Clips A couple of examples: https://adcohearing.com/categories/clips-and-loss-prevention and https://www.amazon.com/OtoClips-BTE-ITE-Hearing-Aids/dp/B00YCK55E0 Try an eBay search for ‘hearing aid clips’. They are usually less then $10.
As you can see these devices only attach the two aids together, which certainly makes them harder to lose – but easier to lose both. For added security you could tie a length of dyneema fishing line (or the like) to the two loops with a micro cord lock attached so that you can shorten the line which you would run around your neck.