[tang daddy]

Ideally when drilling for an overflow 3" down and 2.5" in from the side is safe, you can get glass cut for the overflow box then use black acrylic for the front pane so it's not see through, that is... If you don't want to look at a bulkhead....

I have done a few overflow boxes on tanks and usually make the box 7"x6"x2.5" deep, you want to make sure the box is big enough so the water draining doesn't empty too fast causing it to sound like a toilet flushing. Your return supply should be smaller from the drain. ie. If you run a 3/4 return then the drain should be 1", 1" supply, drain should be 1.25" or 1.5"

The reason behind this is you want to make sure your drain can handle the supply the same size drain may not be able to handle water that's being pumped back to the display causing it to overflow.