Instead of mounting your slider on a tripod the common way, use this pair of Stability Arms from YC Onion to attach the slider more securely. First, tripod-mount the slider, and then attach these arms to hold the slider more firmly. This can be done whether you have the slider mounted horizontally or slanted. When you mount your slider heavily slanted, you can use only one of the arms if you want, attaching the very end of the slider to the tripod, and the other end to the arm.

The bottom of the arms attach to the tripod legs, and the top ends screw into the bottom of the slider. On the leg end, the arms feature a jaw clamp on a mini ball head. On the top, they provide a mini ball head with a 1/4"-20 screw that screws into the bottom of supporting sliders.