Years ago, when the drill motors/transmissions still came with the actual plastic drill casing, we used expanding polyurethane construction adhesive as a bedding compound to form a flat base on the curved case. We put waxed paper on a flat surface, rigidly held the case in the proper orientation, then squirted the glue in the gap at the base, followed by a couple of wedges of scrap wood. Once cured, we held the whole assembly down with large hose clamps. We used PL Premium, which is available at Lowes I think. They have many types, but this particular one will stick to a lot of different materials, including metal. It costs about $3.50 a tube.
http://www.stickwithpl.com/Products/...PLProductID=14
If you use this stuff, do not get any on your hands or clothing!