While a full plate may be the strongest option, a few stiff pieces across the bottom can achieve the same effect with less weight and allow you to see inside and not trap dirt and water. Anyone attempting to modify or weld a critical part of the suspension must know what he is doing or you can damage the part in such a way as to cause it to fail. Better to leave it alone if you don't know what you are doing. The biggest cause of damage to the lower control arm is tow operators hooking their recovery hooks to them and pulling on them to move or secure the car. Next comes hitting things in the road followed by improper procedures removing and installing ball joints and/or wrong ball joints. I haven't seen one rot to the point of collapse, the frame gets there way before the LCA but I suppose it can happen.