During a recent belt change procedure I stripped the alternator inner shaft where you use a Porsche 12 point tool to hold the shaft still while you break the outer pulley nut loose.

I proceeded to take out the entire alternator unit in order to think about my option.  The easiest option was to replace the complete alternator unit, I was able to find a used unit for $250, but for a Porsche dude on a limited budget I was getting some heat from the wife before I even wrote the check out for $250.  So I started to think about other different methods of holding the alternator shaft still in order to tighten the bolt.  What I came up with was to find a allen bolt with about the same diameter as the alternator shaft and to cut the head of the allen bolt off and weld it on the end of the shaft.  Fortunately after visiting our local friendly Porsche dealer parts department I was able to buy an allen bolt that was about 1 mm in diameter larger than the alternator shaft.  So I proceeded to a local machine shop and had the allen bolt head machined down 1 mm.  I carefully TIG welded the bolt head to the end of the alternator shaft, had the amperage on the TIG welder turned way down and wrapped a wet towel around the alternator shaft to prevent too much heat build up.  After I spot welded all around the bolt I hand filed the excessive weld beads off and smoothen the entire shaft so the newly welded on piece looked like it belong there. 

Installed everything back and the welded on allen bolt head held the alternator shaft for me to tighten the pulley nut without any problem.  Test drove the car for 30 minutes and so far so good. 

Here are some pictures of the process.