It may sound odd but have you tried a DIY store to see if they have any clear plastic sheet that you can cut to size, alternatively cheap garden sheds use quite flimsy plastic for the windows so you might be able to cut that down if you can find one. Obviously thickness will be an issue if you want a prefect fit.
Another option is to find someone who manufactures LCD displays the same size and see if they sell a Bezel for it as that may have a protective screen you can use.
If all else fails it may be that the two parts of the plasic at the crack have moved and are at slightly different heights, if so you may find that pushing them back together at the same high will mimimize its appearance, unlikely I know but as a last resort it is worth a try.
Edited by Stephen Moss, Tue May 10, 2011 3:10 AM.