My shifter easily slides off of my desk?

My shifter easily slides off of my desk when shifting gears, especially on 2, 4 and 6. I just bought it used and it only has 2 short clamps at the bottom, but I've seen images of a 3rd thing that you can tighten, almost looks like a screw but it doesn't go in the desk. As of now I placed Velcro with adhesive backing into the small clamp things and the other side of my desk. It holds the shifter but the shifter tilts like crazy and doesn't feel good at all, I feel that the 3rd clamp will help.