Get you a piece of acrylic and drill a hole the size of the humidifier tube then screw the piece of acrylic to the outside frame of the cage. That should hold the tubing just fine. Of course the pvc sounds like a great idea. Run it up the back so you don't see it. I used a piece of acrylic to cover my back and painted it black. I just sit it in the back and have the four corners tied on with black zip ties to the frame. Still allows for ventilation but hides any wires that might show through the plants and keeps more of the light in the cage.