A septum for sealably covering a fluid cavity defined in an implantable device, which may be an access port, and methods for assembling the implantable device. The septum may include a resilient septum body, an annular flange extending radially outward from the septum body, and a reinforcement component disposed in the annular flange for reinforcing engagement with a corresponding annular groove defined in a body of the implantable device. The reinforcement component may inhibit unintended detachment of the septum from the implantable device.