I use a heating cord that I got on petmountain.com, it's a 15w cord. I just taped it to the bottom of one half of my 30 gallon rubbermaid tub with a lid and it stays between 90-95 degrees constantly. I keep my room at about 78 degrees. I cut a hole in the top and put a screen on it, if I see the temperature start to drop below 90 before I go to sleep, I just put some extra egg crates over the screen and it seems to hold a little more heat in.
I keep my tub about two inches off of the floor as well, and you can mess with how much cool air gets under the tub by surrounding the gap below the tub with a towel or something. If I cover the bottom edge on the two longer sides, I found that keeps it a little warmer too. Hope that helps a little!