Sign holder holder, designed to sit on top of a portable clothing rack. The sign holder itself should be available at your local office supply store.
Print, cut out supports with a knife and use. It's fairly big, so it will take a while to print. It may be tempting to set your printer to thicker layers, but there is some more-than-45-degree overhang, so don't overdo it.
The scad assumes that the base is ~96mm wide and ~1.7mm thick. If not, you'll need to use customizer. However, you'll need to rotate the resulting STL so that the flat side is down before printing.