I used a 10 gallon round Igloo cooler for my HLT for a long time, with a 1500 w element in it. It took like 90 minutes to heat up to 180, but it worked just fine! The only thing was, due to the way it's made, I had to go up through the bottom so I had to keep three gallons of water in it (or more) or risk dryfiring the element. It wasn't an issue, since I knew that, and just flipped it off when I was draining.
My element was 110v, and it was the biggest I could use on 110v. You will need a 110v element if that is your power source.