is this tack budget driven?
it would seem mechanically feasible, but unless you're doing every facet of the job yourself (with no dollar value placed on your labor), it looks complicated for what you'll have in the end.
the ladder rack foundation seems very practical. would you consider sheathing the sides with something like 1/2" MDO plywood, well painted (especially the edges)? even if you kept some canvas or ripstop nylon panels, there would be some inherent lateral resistance. as an option, loose pin butt hinges would allow panels to be removed for travel.
assuming the height of the rack (sleeping surface) is 7' off the tarmac, you'd probably want 3' on top of that for sit-up room & mattress thickness. so the top is now 10' high. would the welcome summer breeze turn into a fall sirocco? broadside wouldn't be bad, but end to end wind might be a pain.