[Piwoslaw]

From my experience with rainwater I can tell you that a screen over the top opening is a must. There is always debris in gutters, anything from leaves and twigs to dust. In order to clean out the cube you'll have to almost empty it, and getting anything in through that 6" opening ain't gonna be easy, so the fewer times you do it the better.

Also, algea will start to form in there. I have 4 barrels behind the house, made out of thick blue plastic, and only the first doesn't have a lid on it, and the light coming in through the screen is enough for algea to show up, which then makes its way into the other barrels. So instead of painting it white, maybe you could cover the sides and top with something that doesn't allow light in.