Had exactly the same problem. Just superglue the bit back in with a small bead of glue behind the broken piece. ie stick it to the front casing half. No leak problems as the seal is formed by the rubber pad on the hinged door. The catch and broken bit are not water sealed so only mechanical strength to hold door closed is required from repair.
This catch isn't up to the job and they all seem to do this eventually, rubber bands cut out of 700x23 cycle tubes keep the door closed and the pressure on the seal also.
