Drill a hole for the conduit using a masonry bit. Draw a circle for the conduit's hole, drill six or seven holes along its circumference, then chip out the hole using a small masonry bit. Run conduit to the box using an elbow to make the turn. Tap in shims to wedge the box in place. Stuff newspapers into the cell about 6 inches below the electrical box, then fill the area around the box with concrete to hold the box firmly in place.