I forgot what it is called, but it come with the kit. You put it on when you tighten the screw it expand like a mushroom, against the other side. It fits pretty tight.
For my 2006, it is quite different than momo75 has. Here is the picture taken on the passenger side mounting of the skid plate.There are 4 white bolts on each side and the black bracket. The two bolts that were mounted on the side, the bottom bolt is bigger and fit to the thread on the 4runner...
I paid $100 for it , the saleman gave me discount because I bought a 2006 4runner from him. Normal price is about $170. Dont know if it will fit a 2005 or earlier. Toyota web site has this option for 2006 but did not show picture. This one is original Toyota part, made in Canada.