I molished something similar to re-use the bottle cage seat drillings to mount a tailbox, and then attach the cages (slightly offset, to make room for the bottle) inside the box.
Mk1 sheet of aluminium of the appropriate thickness with some holes drilled in it and some M5 clinch nuts. I added some self-adhesive neoprene foam to account for the slight curvature on the inside of the tailbox, but you wouldn't need that.