I made mine from plastic craft mesh. I found it in the crafts section at Walmart, but I'm sure other craft stores would carry it, too. Since your tank is too small to have the double-divider that I love so much, you'll need to put a lot of plants (real or fake) along the divider. This will prevent your boys from being able to see each other all the time.
Since you used soap, you should do a rinse with a 50/50 vinegar and water solution. I know you think you rinsed all of the soap out, but I'm betting there is still some left in there. It doesn't take much to harm a fish. Next time, just rinse it out with vinegar or baking soda.
Cycling is not just letting the water flow through the tank. Cycling is the process of building bacterial colonies that will detoxify your fish's wastes. I highly recommend having a cycled tank. You'll only need to do one water change per week on the tank. You can read about cycling techniques here: A Beginner's Guide to the Freshwater Aquarium Cycle