I only use a slotted rubber pad when i am thinning the base of a point like a lostlake, calf creek, or pinetree. Or when i really need a long deep pressure flack to remove some junk. The rest of my pressure flaking is done with a leather pad with no slots. I Think your maintain a better cross section when pressure flaking with a leather pad, But the slotted rubber pads have there uses too.
When i use the slotted rubber i just place it on top of my leather hand pad. Never tried a piece of metal, never had a need for it.