In one example, an apparatus includes a casing configured to be coupled to a frame of a vehicle at a first end of the casing. The casing comprises friable foam having a first stiffness. The apparatus further includes a void within the casing defined by (i) an inner wall of the casing and (ii) an opening at a second end of the casing. The apparatus further includes an insert positioned within the void, the insert comprising a material having a second stiffness that is different from the first stiffness.