I installed a door in an opening I have under my stairs. The back of the door is sitting on concrete and the front is on my asphalt driveway. The gap is about an inch. I was thinking of using some type of mortar like you would use on bricks. Maybe use one of those pastry type bags to squirt it in there and not make a huge mess. I don't need much. Any idea on what exactly to use and where to get it? Thanks for any help.