External herbie overflow.. glass thickness?

[JDigital]

I'm looking to add an External herbie overflow box to my tank after some discussions with a few people regarding sumps (which I had originally planned on omitting from my build)

How thick does the glass need to be?

I have some left over glass that originally came with the tank for a glass top... but it is prolly only 5-6mm thick. Would this be thick enough or will I have to look else where for thicker glass? (and how thick?)

EDIT:
I'm looking at about 9" tall (half my tank height), 16-18" wide, and 6" deep.

Two 1 or 1.5" holes in the bottom for the drain lines. (haven't decided yet)

TIA
Josh