Here's a pic of a sump I just built, you'll notice that there are extra bits.... I didn't plan it out that well (I thought I did but I was having a brain fart) it's water tight, 24x14.5Hx11.75 wide. You may also notice that it's got a pan (drop down area) that's where the water will sit while waiting to go back to the tank. The Kent float valve will be hooked up to the Kalk reactor in the next pic.
This is a Kalk reactor I built, you may notice the tubing going in at the bottom, that's where the water from my supply (jug) goes in and it flows out the top as needed for auto top off. The blue air valve is there to help remove air that gets into the container.
Doug