Amazingly, my Kona bike has completely unsealed top steering head bearing, with the part line on the top surface so the rain just runs straight into it. It has a useless plastic dust cap, but no, it doesn't keep rain out.
WTF Kona? Really?
This rain cover sits under the spacer rings, i.e. it is held down by the spacer rings. I remade in Nylon as the original ABS part started to show cracks after a couple of years.
It is useful to make a 90 degree segment first, to check the fit against the curves, before making a whole ring.
Parametric, so you can change it to suit your bike if different.
(Note: Thingiverse customizer only shows full part, maybe an old openscad version that doesn't accept angle param to rotate_extrude)
Designed to print cup-upwards without support material