What does the temperature look like with what you have now?

For starters, put some sort of insulation around three sides of the tank, plus the bottom. Foam is good insulation; if you use foam core (like presentation boards, etc) you can maybe make it look a little nicer than just foam insulation board. Wood is a good insulator, too. I have a 80-watt RHP on a glass tank (and the screen lid mostly covered even where the RHP doesn't cover it), which keeps the temperature very nice and even. I am completely an RHP believer, because it works so well on a (insulated) glass tank even in a room that can get below 55° in the winter.