Paper milk and juice cartons are great for starting seeds because they are waterproof and strong enough to hold soil. You can cut the carton in half or make a window on one side, fill it with potting mix, and plant your seeds. Home Baked Bliss recommends cartons for bigger or fast-growing plants. When you're ready to move your seedlings, just cut off the bottom and plant the whole carton in the ground to protect the roots from pests.
To really maximize the effectiveness in warding off squirrels, use two pie tins. Put one on the top for the impact of the shiny metal and noise and one under the feeder to literally barricade climbing critters.