Avoid a hard plastic clip-on case of any type. The clips that hold the case onto the MacBook can generate pressure points on sensitive areas of the casing, especially the lid/screen. This can cause damage to those sensitive area. And while the case may clip on somewhat easily, it can be difficult to remove when the time comes for cleaning or otherwise. This especially affects (again) the lid/screen assembly which is pretty delicate to begin with. Also, dirt, dust or grime can enter between the case and the MacBook itself and act as an abrasive causing other damage.
Just stick with a padded, zippered sleeve/case as was suggested above by Chattanoogan.