Will try and cut on the factory cover/rubber boot and see if we can make those work as i have a ruined one.
While the aftermarket dust covers will provide a tight fit, you may not be able to hand cut the hole larger in the stock cover and get it to seal well against the body of the lamp base. Unlike a projector bulb converrsion, the LED retrofit still relies on the reflector to focus the beam. I disagree with the position that it is to keep rain out and that there is not much rain up there. It is called a "dust cover" and I believe that if you don't get a good seal that you would have a host of undesirable things inside the reflector including dust, small bugs, condensation, etc. I can't think of any hidden open place that stays clean.