I would recommend instead inserting a solid spacer between the existing foam pad and the base of the shoulder rest. You really don't want a thick "squishy" pad as that will tend to change thickness as you move while playing (not to mention collapsing over time as the material ages). Another alternative is to use any kind of spacer and put a layer of the thin "fishnet" anti-slip rug pad material you can get most anywhere.
However, I'm concerned about your posture in the first place. If you have the violin properly positioned, the shoulder rest should not be prone to slipping. You may want to have someone evaluate your overall positioning of the violin relative to your chin and shoulder.