What I propose is a lightweight molded packing space that is just less than the size of a car's actual available packing space (these tough plastic shells would be orderable by mail), so all that arrangement and rearrangement can be done inside, while the packers remain unassaulted by weather or ruffians. Once you've come up with an arrangement that seems workable, you can take a photograph / make a sketch, and then efficiently recreate the arrangement in the actual car.

In the alternative, if the packing shell (and the packers) were strong enough, the thing could simply be strapped / zipped / plastic wrapped to prevent spillage, and the whole thing hoisted out to the waiting car, slipped neatly into the space it was designed to replicate.