Ideally, that would have been part of the mold. If you look at a sink it normally has a passageway to overflow holes. What you're going to end up with amounts to a bucket with a hole in the bottom. You'll need some method of cutting a pretty accurate hole. It would be helpful for you to know the thickness of the base and compare that to the threaded portion of a bath drain. If you want to make it really neat and correct it should have a slight bevel around the hole so you can get a good seal when the drain is installed. Just study how a bathroom sink is made and you can tell how the drain hole should be formed. If you're really good you'll figure out a way to get a drain pipe in there that will seal (maybe a spud type attachment) and only concrete wil lbe visible on the top. I think it would be cool for the drain to be opposite side of the wall and not visible at all.