The hole can be from 9 to 12 inches above the floor. You can put an inch or 2 of sawdust, woodchips, dried grass clippings or fine chat in the bottom of the box. Sparrow hawks don't build a nest as such .
Live trees are least desirable because racoons and snake predation on nests seems to be worst there.
Use only aluminun nails in live trees. Locate the box in open or partially open land or at the edge of wooded draws or fencerows which are adjacent to pastures, hay meadows and roadsides. Try to locate the box within 15 to 30 yards of a tree with dead limbs, a snag or a power pole. These serve as a plucking perch where the male birds dismemeber their prey and pass the food to the female, who feeds the young. These perches are also used for preening, courting and as perches by the young when they learn to fly.