I try to keep umbrellas handy, and I bought several little waterproof boxes cheap on amazon. They are just about the same size as the little pocket case that came with my aids, except they snap closed with an o-ring seal. They are just big enough to fit three hearing aids in, so just a little too big. I have one in my car and one in my desk, etc.
Not long ago I was on a ‘camping’ trip in my RV. We had biked a few miles away and storms started brewing. I had no such case or umbrella, so I tried scrounging up a plastic bag. Could only come up with a piece of saran wrap from the kitchen at the place where we were! I wrapped them as best as I could and in the pocket they went! All was good!