As a hack for gaming devices such as yours, create a high-performance non-slip 'gaming' mat which sits on the surface of your desk.
The problem with small rubber feet and anti-slip materials is that dust, dirt, crumbs, body oils, etc. soon make both surfaces slippery again. They're not easy to maintain. As you've also discovered, there isn't much surface area touching the desk surface for the pads to be optimal.
This hack is different.
The base of the removable mat is large enough so that you hold it from moving with the weight of your arms, keyboard, monitor, and maybe the computer itself if it sits on the desk. A sheet of thin, smooth plastic laminate would be ideal. Pick a nice pattern and ensure the edges are smooth and burr free.
Purchase a container of Sugru - a mouldable glue (it comes in light and dark grey and some colours) which turns into rubber after curing. Check the Best Before date on the bottom of the package to ensure best results. You can find the stuff at some hardware stores and online.
Decide the best position for your tracking device will be. Position the trackball.
Roll the Sugru into a long strand (6-7 mm/1/4 in.) and wind it around your track ball to form a tight fence to hold your trackball in position. Push the Sugru down to stick securely to the gaming mat base. Avoid gluing your trackball to the mat so that you can easily remove it to replace the batteries and also to clean up under and around the trackball when you finish a pizza, say.
You may wish to 'line' the bottom edges of the trackball with removable 'magic tape' to lessen the chances of getting the glue on it when you strive to get a tight fit.
Let the Sugru dry, bond with the mat, and become rigid and permanent. When it does, your trackball will not slip or slide during the most demanding gaming session. The Sugru will take approximately 12 - 24 hours to turn into rubber.