This handle has a slot that fits ordinary corrugated cardboard, like from a shipping box, and holds it quite well. You can secure the cardboard panel to the grip through provided holes using a zip tie or wire or string. The closeup photo shows a zip tie holding the board.

Two STLs: The basic model holds one piece of board. 

The other STL holds two panels and would be useful for a two-sided sign if your cardboard supply has only one blank side, or if you want a super sturdy sign (tape the boards together).

Prints strong in draft mode and PLA (15% infill, 0.3mm). After printing, clip off the stabilizing feet.

Be seen! Stay peaceful.