For the most part for what you are doing.. only common courtesy says you have to put anything on it .. but as Nos said elsewhere recently, grab a hi viz vest and it would be fine. Your objective is just to warn that you have a protrusion from the end of your vehicle/trailer, but you are allowed a good amount before special markers are neccessary .. and if you cant find a Hi Viz vest .. speak to a builder .. cos right now .. they dont need theirs as much
Ofcourse all IMHO last time I checked, but everything changes rapidly these days, perhaps someone else can confirm