Sorry, but this is probably a non issue. The Plexiglas cover is held on by magnets and is easy to remove / adjust. The bottom is held in by tabs that hold the monitor in place. The top is held in by magnets. So........
Grab the lip that is hanging over (at the top) and pull it back towards you. As you are doing this, grab the upper edge (as you go) as it peels up from magnets. Once the top edge is completely pulled away from the magnets, just adjust the entire cover towards the short side a little bit and set the upper lip back down.
If that does not work, then the mounts are off either on the cover, or on the iMac. You can return it if it bothers you that much.