Many manufacturer's of shaft mounted gearboxes offer standard guards for piggy-back applications, like screw conveyors. You might be able to find one close enough for your reqyuirement. Check with Martin Sprocket, Falk, Hub City, or any of the many others. Otherwise, there are a lot of machine shops that can make them with a few critical measurements, shaft diameters, center distances, pulley diameters, pulley width, and amount of take-up required.
Russell Giuliano