Nisi makes a 100mm filter holder specifically for the Nikkor 14-24mm f/2.8 S lens. This 100mm filter holder does not vignette at 14mm with two filters in it, and rotates to allow for positioning of graduated filters. Either 100x100mm or 100x150mm filters can be used.
Haida makes a rear lens ND filter kit for the 14-24mm f/2.8 S lens: