From Big Dragon we have their Flip Up Lens Protector in Black.
This is a must have accessory when it comes to protecting your Optics! Mountable to any standard picatinny rail, this protector will sit in front your optic and stop any unwanted BBs from meeting your optic. Comes fitted with a Clear Perspex Lens as well as an additional Yellow Perspex Lens. The Base is constructed of metal.