Questions about enclosure thickness.

Hey all! I am building an enclosed grow box, and I am putting a 400 watt HPS in it. I was wondering what shoulod the thickness of the glass be?? I am not really sure how thick it should be but too thick and it will diffuse the light right? and too thin, it will transfer too much heat...I think. How do I win? Thank you guys!

going to say 1/8th of an inch thick on that one

though i havent done the research, any thicker logcially seems too thick.

it doesnt relli matter if it causes the light to disperse as the plants will use it neway. jus sayin guys