Try attaching a couple of 3’ pieces of 1.25” PVC to your table legs using hose clamps or even duct tape, slide 3/4” PVC pieces with “L” fittings at the top inside them, another piece of 3/4” PVC cut to fit between the L’s and you have a simple 3 piece frame from which to hang your sign. If the 3/4” PVC flexes too much, slip some dowel rod inside to stiffen it.
Never actually tried this, but just thought it up and it seems it should work!
Gary