Everybody has the problem and there are a lot of solutions to tightening up the light housing, I elected to shorten the screws so it would tighten up properly, but there are other easy solutions, some that are mentioned above and only limited by your genius.
When you get the light tight on the fender, you bulb will still burn out pretty frequently because of vibration. Buy and LED bulb and be through with it. If you read this forum, you will see lots of uses for the extra watts left over by the use of the LED bulb. One of the advantages is a better headlight at low rpm. The LED bulbs are available in clear and white at your favorite auto parts store, and I think the number is the same as the factory light.
Ride on--John