I had to do the same thing to the back of my cargo carrier.
Wasn't me forgetting it was there. It was the people who pulled up behind me at stop lights and 'then' forgot the carrier was there because they couldn't see it once they pulled up behind me and then they would move up and hit it.:R
I used the
red rigid driveway markers
. Look for the ones with a flat or four sided pole. They stay put better no matter how you attached them.
I then spray painted the pole black so it didn't look so tacky. Even took a brush and painted the trim around the reflector black too. And just left them on.
BTW: When parked at CG's, before I put the reflector poles on it everyone was walking smashing their shins into it including myself.

And get a roll of this and put it on the sides so at night it can be seen.
