I bought a bag from baboon to the moon. It works as a backpack or single strap. It's waterproof and right now orange/red/blue are on sale (i have the 2nd largest one and just recently saw their sale and bought the large small and extra small ones).

I primarily use it for sand volleyball but greatly prefer it to using a backpack or any gym bag i've had before.

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this: https://baboontothemoon.com/products/go-bag-mini-duffle?variant=32701825908807#variant=yellow