Maybe I'm confused, but they are designed to overflow - into the aquarium. If you are indicating that they are spilling out over the back of the filter and not into the aquarium, then the mounting to the aquarium might just need to be adjusted. As I recall on the aquaclear (never used a whisper), there is a piece of plastic that can be cut to adjust the angle at which the filter leans forward into the tank. If it is not long enough, the filter will spill water out the back of the filter. You can always improvise by making a little prop to fit between the back of the tank and the bottom of the filter box that effectively causes the filter to lean forward. You could use anything for the prop such as a piece of 3/4" tubing or a straw. All you need to do is make sure the filter is angled forward enough to spill into the tank.
I may have missed the point of your post, and if it is spilling do to the filter pad that you placed at the exit of the filter, you might just have to remove that and keep your water level high enough to prevent the turbulence.