I believe that the 'wart' around the drain plug is just looking for a rock to snag onto & halt ones forward progress at the most inopportune moment possible.
A skid plate ought to be as smooth as possible on the bottom & give any drain plug or engine case assembly bolts at least 3/8" of clearance. Even if the owner/mechanic has to install home-made rubber donuts hole-sawed out of old mud flaps or tires to pad the frame rails! (Like I do on the Ricochet skid-plates.)
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