This design would not be any harder to get on or off than a clip on
type.  You would use the same mounting points as the standard rack, two
screws on the front, two on the side, and four nuts on the back flange.
It would definitely take two people to deal with it, or a hoist system,
but that would be the same as for something you set on top of the rack
instead of replacing it.  This replacement closed luggage box would be
simpler, stronger (one piece), more aerodynamic (with no joints), less
susceptible to wind driven leaks since there are no joints, and give you
more space.

I converted my '68 Westy years ago to a custom high top and really loved
that over cab storage.  Even traveling with two kids under the age of
six we never filled it all up, even for three week trips.  Maybe what
you really need is an Adventure Wagon, or an Adventure Wagon top!

I don't take as many trips anymore, but will again someday, and if I
could afford the Adventure Wagon mold I would buy it and convert my '84!

Stuart