Simple shower head, standard 1/2" inch thread. Widen the holes with a drill if water pressure is too high. Print it hot so layer adhesion is high (you don't want it flying towards your face when you shower).

I included a small print with the same threads as the shower head so you can test the fit. It is okay for the test piece to be very snug because you will have more leverage twisting the shower head in as it is bigger.

Fun fact: Holes are modelled with prime numbers in mind.