Build a rectangular frame out of 2-by-10 boards. You'll need two boards that are as long as your desired height for the clock, and two shorter boards for the desired width.
Cut the canvas or paper so that it will cover one open side of the box; this will be the front of your grandfather clock prop. Use the gold and black paint to add such details as the outline of doors for the front cabinet, the pendulum and the face of the clock, including the numbers and hands. If you want a partial moon dial to appear above the face, you'll need some blue paint as well. Give your paint 24 hours to dry (or longer, depending on the paint manufacturer's instructions).
Hammer tacks or nails into the paper or canvas to secure it to the clock frame. Canvas is more durable than paper and is affordable for all but the most basic of budgets.
Set the clock upright and test it for stability by touching the sides, applying gentle pressure. If you're concerned about the balance of the clock, place a brick or cinder block inside the bottom of the frame to give it extra weight.