Aluminum Gavel Train Box - Door seal

[Portable Welder]

I have a customer that I have to install a rubber seal on the rear gate of the gravel train box, He says that it has to be water tight.

The rubber that I am using is 1/2" x 2-1/2", I'm going to make a track to hold the rubber out of aluminum flat bar.

The question that I have is how much should I leave sticking past the door. ( For the smash. )
1/8", 3/16", 1/4", 5/16" or 3/8". --- Maybe 1/2"

The rubber was supplied by the customer and I dont know what type it is, I do know that its stiffer than skirt board rubber, I'm going to say its about as stiff as conveyor belt rubber that a cement plant would have.