The "nerd leash" I use for outdoor activities is made by Chums. It's the kind that has a silicone/neoprene sleeve that you wet and slide your frames into. Mine also has a round cord lock that you can adjust how snug you want the glasses to fit?

I'm sure you could come up with something similar, out of shock cord, but it "might" be more effort than it's worth considering the cost of the Chums :-/

Check these out, they have what you need and even if you decide to try to DIY, you’ll get some good ideas.

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