I'd recommend a TBS Unify 90° MMCX-SMA pigtail and a custom 3d printed mount - there's a ton of designs available for the Source One. I'm partial to the Unify pigtails because they actually have a threaded flange that's very easy to mount.
Either that, or get an antenna with a solid stem and a direct 90° MMCX connection
@Mikey the same thing actually happened to me, I was able to get the cap to lay on top of the vtx, could you rewire so that the the pins are long enough?