The gnarliest racks commonly available are called "Safari" racks and are
made by a local San Diego outfit called 'Confer' (or Kon-Fer or
something close). The offer two attaching options, one that attaches to
an existing wimpy roof rack, and one that clamps to rain gutters. You
have to install the floor of your choice as it does not include one.

These are what Marlin Perkins  would have on top of the Land Rover when
driving through Africa carrying extra spare tires, gas cans, ice chests,
etc.

Outfits in San Diego like "Offroad Warehouse" 619 565-7792 order them
for you in the configuration you specify (width and length).

DDF