Anytime you remove a bolt you need to put a spot of blue lock tight on it then torque to specification. That looks like a water return with the alternator bracken made as a one piece. I would try a dismantler for the part.
I checked but could not find the part number, but it will be much cheaper from a dismantler.
http://www.partrequest.com/used+auto+parts/make/porsche/boxster