String Trimmers

I just bought a HU625WT wheeled weed trimmer mower. It worked great, but it also managed to launch piece of gravel 50 ft up to the second story sliding glass door, shattering the outer pane of glass! Is there a larger deck accessory item available for purchase that I can mount on the mower? That way I can contain these projectiles, so I do not have to worry about this happening again in the future. Thanks!