Today I came across this random SLAM just hanging around in thin air. Someone also decided to place slams on shop and bank doors, and in the fountain. Very strange. Uploaded a demo for anyone else to see: slams.zip
Old trick. Just place a prop, or even a stack of props first (like a box, barrel, etc..), and stick the slam to it. Then remove the prop with a grav gun from behind, so as to not trip beam. Slam will remain attached to the previous coordinates, and will remain fixed there even after the prop is removed. Enjoy