Planning to build this out of a 10 Gallon Tank. The inside/front of the tower will be open so the trays can slide in and out. I will use acrylic sheeting to build the tower on 3 sides. I will glue acrylic strips/gliders so I can slide the trays in and out. The spot for the Bio-Balls will have a acrylic divider to the top of the tank, then the tower above that will just be 3 sided. The top of the tower will be another sheet of acrylic, that is where I will screw in my inlet. I do not think I will have an overflow issue because the inside/front of the tower faces the 10 Gallon Sump so the water could just overflow if the drain holes in the trays become clogged.