-1 cabrios can't fit everything. I couldn't get between the roll bar and the windshield or the roll bar and the back gate or under the roll bar. The one box I was trying to move had to wait. That box was much smaller than the treadmill.
If one left the gate down and bungee-corded the box to the roll bar, covering the brake lights with this "behind the car" carrying, perhaps, but no - too dangerous.