External herbie overflow.. glass thickness?

[sphelps]

Both 5 or 6 mm will work just fine for the dimensions you stated. Be sure to use the right silicone, GE Door and Window 100% Silicone #1 works well.

For the holes you'll be fine with 1" drains for a herbie style, but use the bulkheads that require a 2-3/8" hole as you can get 1.25" Bulkheads that fit in the same size hole.