Traction battery packs are disclosed for use in electrified vehicles. An exemplary traction battery pack may include an outer enclosure assembly establishing an interior, and a battery array housed within the interior. A polymer-based enclosure cover of the outer enclosure assembly may be structurally reinforced by one or more structural cross-members. The structural cross members are designed to provide structural integrity during both normal operating conditions and thermal/venting events of the traction battery pack.