DO NOT USE SILICON. Sorry for the caps but I need to stress it. Silicon tends to evaporate and fog up your headlight. It may not do it right away, but months or years down the road. After it dries up, it leaves deposits on the top of your headlight lens.
What you should do is (this is what all retrofitters do):
Put half the headlight back in and bake for about 5 mins with the oven off, after preheating it to 300 degrees F. Then clean off all of the factory sealant while it's still hot. Go to your local auto part store and buy 3M Windo-Weld. It's $18 for 15'. Put a bead in the channel of the headlight, heat it in the oven for 5 mins with the oven OFF after preheating it to 300 degrees F. Remove and press the 2 halves together and hold with gator clamps (Home Depot
, $1 each). Should cure in 30 mins and good to go!