Radiant heat panels are great, too. In order to mount them to a screen top, you have to put a piece of wood (it does'n't have to bea big piece) on the outside to have something to screw into. But you can use a RHP for the ambient temp and a supplemental heat source such as a UTH for the hot spot.
The other reason a RHP is efficient is that the whole thing is basically inside the enclosure, and you can insulate the top above it. That means you lose less heat to the rest of the room than you do with a CHE in a fixture sitting on top of the screen.
RHP's are sort of pricey initially, but it works out to be way cheaper in the long term (not even that long) than heating the entire room with a space heater.

And also, insulate your tank the best you can. Foam insulation board is great, but isn't the only option. Several layers of corrugated cardboard will help, various kinds of packing foam, quilts/blankets, etc. That includes the bottom of the tank, since it will lose heat to a cold floor. Insulation makes all the difference in the world.