I wouldn't drive very far or very fast with the rear hatch tied down (manual doesn't recommend it at all).
But for short, local trips just cut yourself an 8" length of 1"x2", 1"x1" or other suitable material. Tie your line around the middle of it and cut to length. Make it long enough to go from the trailing edge of the hatch to the metal latch at the center rear of the cargo bed.
When you want to move an oversized item, pop off the hatch handle (little plastic recess used to close rear hatch) insert stick so it lies parallel to the drip edge and is supported on either side and tie other end to lower latch. Make sure your stick is long enough that neither end can slip out.
The plastic cover is easy to remove without tools. With the hatch open, catch two or three fingernails from each hand behind the long edge of the cover closest to the exterior side if hatch. Apply pressure towards the recess to flex it slightly away from the little catches and pop it off. To insert just pop it back in after making sure it's not facing the wrong way.