Although I agree with robocop's sentiment I'll go ahead and play.
They use to make laser sights that would mount on the trigger guard. This was back before everything had a rail on it. Not sure if they still make them and I always thought the mounting looked questionable, but that may be your best option if you're determined to do so.
"For every problem there is always a solution that is simple, obvious, and wrong." - Mark Twain