Landrover specify that the Expedition Rack (for the 2020 onwards model of Defender) isn't compatible with roof boxes, but this may just be because they don't have any boxes to fit. We just have a few boxes that will work with this rack. You will need to go for a box that’s small enough to fit within the edge of the rack and also a box that has a flat base. Many boxes have recesses on the base that accommodate two cross bars, these won’t fit your rack because the extra cross bars will clash with the deeper section on the base.
The following roof boxes will fit the Expedition rack:
Hapro Roady 350 roof box, anthracite black, no. 33191. (with additional T-track adapters)