More information would be helpful. You didnít indicate the current depth of the distribution box (from the surface of the ground to the top of the box) or the material from which the box is constructed. Is this information available? If the distribution box is pre-cast concrete with a concrete lid and buried under two feet or more of soil, the weight and vibration of a typical riding lawnmower shouldnít have any adverse effect on the box at all. Depending on the depth the box is buried, there may be no need to construct any sort of protective mound unless itís to keep much heavier equipment than a riding mower from being driven over it.