when i fill those holes, i make a cardboard pattern that fits around the bottom side [doesn't matter which side, as you will do this while mount is on it's side], then i cut a piece of wax paper to fit this pattern. tape the wax paper/cardboard [i just use a couple of pieces of double side tape] pattern to the outside of the mount, lay it on it's side, and fill the holes. let the urethane cure according to the directions. the wax paper then peels off the bottom side of the urethane, leaving a smooth surface. makes quite a difference in the mount "stiffness". if you don't have any wax paper, other stuff to use is a thin coating of wheel bearing grease, silicone grease, etc. the object is to have something the urethane will not stick to when it sets up.