On this parts fiche the Collar is the most specified component.
A sample from the products for the Side Covers include: Cover-side,rh,s.white, Cover-side,lh,s.white, Collar, Bolt,6x16.
The most reasonably priced component used on the Kx450d6f Kx450f 2006 Usa Canada Side Covers is the Bolt,6x16.