Holabr, here is the correct answer. Yes a proper cover is very useful. That is a form-fitting vinyl cover that your local dealer can order. The cover should leave the bottom inch of coil uncovered for air circulation. Aside from keeping out various debris, the cover provides an essential function: ice damage protection. For you southern boys, here is how this works: Snow falls and drifts into coil fins. Temperature increases during the day. Snow melts and wicks between fins. With dense fins, water will NOT simply drain down, especially if fins are dirty. Temperature drops at night. Water freezes. And what happens to water as it freezes? It INCREASES in volume. Put a full bottle of water in the freezer. It will break. Now you want to guess what might happen to the condenser fins?
All of this assumes the condenser is elevated, so the bottom of the coil doesn't sit buried in snow for 3 months.
For the fall, a screen will work, but you need something a little more secure than ordinary magnets. If you have access to some really strong, rare-earth magnets, you're in business. Yes the screen will restrict airflow just like a dirty coil will, so you will lose some efficiency. You might want to use a screen with larger "holes" than window screen.