I mounted mine on to happy trails racks
but mounted them with the tops to the racks.
i drilled some extra holes in the racks and use some aluminum flat bar inside the case to hold it better.
i put the larger side on the outside, so the big part opens and your stuff doesn't fall out allover the place
I used some cord from blinds (very strong) and loops to hold them open.
eagle creek single/ double cubes fit exactly into the case.
also put a tool tube on the end of one case
Because a thing seems difficult for you, do not think it impossible for anyone to accomplish.