I would imagine its not been done too many times. If you are wanting to add a sump, why not just get a different tank and get it drilled for a sump? You will save a lot of money. The whole purpose of the nanocube, is that is entirely contained in that one box.
Sorry, I'm not really helpful, but in my opinion, if you want a sump, you should get another tank and get it drilled, or even get a custom builder to make you one already drilled.