My current outdoor hvac unit is mounted on a plastic pad that is partially embedded in the patio concrete. The patio slopes away from the house and 1.5 inches of the rear of the pad is in the concrete. I need to install a larger pad for a new unit but not sure how to level the pad on the sloped concrete. I plan to remove the old pad and put the new pad over the hole that will be left from removing the old one but since the new pad is larger it will sit on the sloped concrete. Any suggestions on how to level the new pad?