The loss of the rear reflector is a common problem for the RS. There are two causes. The screw can loosen and come out, or the plastic under the screw head can break away, resulting in the loss of the lens, but not necessarily the screw. The loose screw problem should not be addressed with Loctite. Loctite crazes plastic, and will hasten the problem with the plastic breaking under the screw head. The screw cannot be tightened solidly, because the lens never bottoms out. If tightened too much, the plastic breaks...Catch 22! A dab of RTV silicone on the metal tab where the screw attaches, over the screw hole, before the lens and screw are installed, is a good fix. Be sure to let the silicone cure overnight before riding. Adding a additional few dabs of silicone where the lens meets the housing is a good insurance policy, too.
The lens can be reinforced at the screw hole with a small piece of styrene plastic from the hobby shop. and some plastic cement. Model airplane cement will work. Glue a piece of heavy styrene (or acrylic) on the back (inside) of the lens, over the screw boss. Let it cure overnight, then drill a hole to match the existing hole in the lens. Find or grind down a small washer to fit inside the screw boss, beneath the screw head. This should be as large as possible, without hitting the edges of the screw boss. Use the silicone on the housing tab's screw hole, as described above, and just snug the screw down. Too tight will break the plastic. Let it cure overnight, and you're in business for a long time to come.