Don't have to turn off hearing aids. Just turn volume down lower that will not cause feedback or try to loose cap made a bigger size for both ears n' devices.
Here are the sample of winter hat webs:
http://ecx.images-amazon.com/images/...6L._SS400_.jpg
http://ecx.images-amazon.com/images/...UL._AA280_.jpg
http://ecx.images-amazon.com/images/...HL._SS500_.gif
http://ecx.images-amazon.com/images/...XL._SS400_.jpg
http://ecx.images-amazon.com/images/...GL._AA280_.jpg
I like this kind of material better for bte hearing aids becuz of the switch will not stick together into material if u choose crochet yarn, it may have bothersome.