Best way to make a shelf "in" the wall in a shower

I am about to put walls up in my renovated ensuite. I have having anything hanging off shower taps in the shower to hold shampoo bottles etc, and I hate keeping stuff on the floor. I want to put a little shelf/cavity "in" the wall to hold shower bottles soap, etc. My question is, what is the best way to do this? Doing this obviously creates a potential leak point, therefore it needs to be done properly. I am thinking of getting a waterproof sheet metal frame made that I embed within...

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