Looks good. I wouldnt worry about downsizing the water bowl. Just give the cage more airflow by doing what you did and uncover the top a little. You can uncover a little bit of the warm side and the cool side. That will give you an air current per say. The cool air from the cool side will settle into the cage, move across the floor and the warm side will raise the air up and out. Play around with the CHE placement until you get a nice temp gradient of 86-88 on the hot side and 77-79 on the cool side. In a 20 gallon tank, that might be a bit hard but it can be done.