I made this product to hold an aquarium float while filling my tank with reverse osmosis water. The tube is about 6-7mm wide or a ¼ inch wide and fits snugly. I print this with no supports.

The way this is designed it should flex slightly to snap onto the molded plastic on the side of the top of your tank and not fall out.  I used this float from amazon to build the rest of the water stopping feature you can see in the photo. 

Let me know how it works out for you or if you have any suggestions for more makes!