>>349872372
As for the Index, this one was the one suggested and it's been absolutely great for workouts and such. Not the greatest for long term comfort though, but I swap back to a normal gasket once it's over. 

https://www.amazon.com/gp/product/B08LNBYCN1/

As far as for other headsets, I've tried a silicon gasket for the Quest 2 and it's pretty awful for anything physically intensive. Caused me to sweat more profusely and got in my headset and eyes more often. I'd recommend looking into any washable cloth ones if you have something non-index related.