The only z4 spoiler I ever saw and liked was from a company called MS
I dont know if it ever went into production or that it was a design study for a show or so (as you never see it IRL or in shops)
All the other rear spoilers are pretty ugly and dont suit the design of the car imho, even the alpina one.
this is the MS spoiler; I dont think it would be hard to reproduce:
But it has been made, this is a picture, not a render.
This is the one I would like to see made/sold