6mm graylite black (actually dark gray) or other dark tinted glass to see how much a glass internal overflow box would cost. 4 pieces total
4 inches tall on the sides,
3 inches tall in the front (to provide for water to fall over the front edge),
36 inches wide,
1 inch front to back (bottom piece).
Then I'd probably get a clear glass box with 4 sides, dimensions of 5"x5"x36" to silicone behind the tank and I'd cut 4 1.5" holes in the back wall to link the internal box with the external box where the plumbing actually is.
I've contacted Adam to build me something nice out of acrylic to cover the plumbing part in the back since it won't be in the wall.