When I first saw plans for a refugium in the back of a nano cube, people modded small lights in the hood, so the light same from on top. I really wanted a refugium after my last nano attempt was overun with nusience algea. However, I did not really want to put the time or money in to modding lights in the hood.
A peron at my local fish store told me that the black part of the nano's back is not actually plastic, but glass covered in black tape. He said he put his live rock rubble in and made intricate designs visible.
I decided to try using a standard 40-watt 2700-k light clamped on to the tank and shining through the side. After dumping in less than a handfull of caulerpa, and a piece of sponge that came with the tank to hold it in place, I waited to see what happened. With these weak lights on for about 15 hours a day, I was not expecting much. Now, at four weeks later, the caulerpa has more than dougled its size, and my nitrates have dropped.
Here are pics-