Doc Willie - Do you have a pic or depiction of what you ended up using? I was going to suggest something more like a frame with a groove in it to support the bottom of the two signs and the top of the signs. Then run a rope from the frame into the trunk - just put a big knot on the end of the rope and close the trunk. The frame could be rubber/plastic or have pieces of sponge or similar material to create a soft backing against the metal of the hatch.
Dual Lock comes in strips about 4 in long and 1/2 in wide. I have two strips, end to end, vertically on each side of the sign, which is 8x8 inches. Total of four strips for each sign. Then there are four strips on the vertical part of the hatch back, next to the license plate, placed to match those on the sign. I have a set up like this on either side of the license plate. So there are four vertical strips 1/2 by 8 spaced across the back of the hatch. If not clear, I'll try to get pictures tomorrow.
I’m running across this issue now. Magnets I bought cling to the doors but not the trunk or rear bumper.
Whatyaknow, just checked: frdige magnet doesn't stick, on our '10. In British Columbia, we have a graduated driver's license, with new license holders required to afix a magentic sign with an L (Learner) or N (Novice) to the back of the car, somewhere "visible". Tuck it in the back window I guess.
I was in a funeral procession a while back where the funeral home passed out little purple magnetic flags to identify the cars in the procession. Most were sticking them on the hood. I tried that and quickly remembered "right! aluminum hood". Finally found a little bit of usable steel next to the glass on the roof.