How to make holes (and pattern them) along a curved surface?

Hi all,

I've been trying to design something like a shower head, where I split a single flow of fluid into many smaller channels.

I've gotten it like this with the curved part all hollowed out on the inside, but I can't seem to place the holes. I've tried using hole wizard but I keep getting an error.

P.s. the rectangular piece is just representative of the workpiece that I want to clean.

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