Your uth should basically cover approx. 1/3rd of your tank floor surface. You can use an assortment of things to help redirect the heat. Insulating board that they wrap houses in before putting up vinyl siding, tin foil, plastic packing styrofoam sheeting... just take a tour of the insulation isles at home depot or lowes. I use the plastic styrofoam (it doesn't crumble when picked like the stuff that comes packed around stereos) because it is poorous, it can breathe so the uth isn't over working, and it provides a great barrier to redirect heat upwards imo.
This is what I use on 4 sides of my tank, both sides the back half way up, and under the uth on the hot side. You can see in the third pic of the tank the insulating on the outside of the tank.