A self fixturing sidepack cabinet for a utility truck body has right and left doorjamb components that are positioned to the right and left of the cabinet door in the closed position. The door closes against door stop components positioned around the doorjamb, and extending inward. The door stop components may have a flexible edging component fitted on them around the doorway. The top door stop component extends from an upper door frame component configured with tabs bent 90 degrees from the door stop surface. The tabs slide behind the left and right doorjamb components to ensure the top of the door opening is dimensionally correct and installed at the proper angles.