I'm looking for a 3D printed antenna holder that can be rotated around the horizontal axis, allowing the antenna to change pitch angle, or an ad-hoc hack to achieve something similar.

Would appreciate any links or ideas

You could achieve this by using 2 45 degree SMA adapters, and rotating them each to get your desired angle.

I haven't seen an adjustable mount, normally they are at 45 degrees which is close enough.

You didn’t state what antenna you want the mount for. If it’s for a ‘Immortal T’ style I use these and they work great:https://www.brain3d.co/collections/hd-an...7495701591