Lids are a good idea to keep varmints out or to prevent snowfall from filling your compost bin in the winter.
You can easily add a lid to your compost bin by building a frame out of treated 1 x 3 or 2 x 4 treated pine. The Lid can then be covered with a plastic tarp or other waterproof material to keep heavy snowfall, critters, (or kids) out.
Lastly, add a pair of hinges to flip it out of the way when accessing compost.