You can see why you were having chatter on your light weight flywheel from about 9-11 O'Clock'
You are lucky that your mechanic knew where to get a Porsche guibo (or Giubo see it both ways) disk. Porsche only offers the whole shaft for $$$$.
These folks have them also. Many folks have successfully used a BMW guibo. I think it is marginally thicker.
"Driveshaft flex disc “Guibo” - C4 and Turbo
These folks list the OEM Porsche replacement for $84.06
KO Performance in Florida
KO Performance - High End Import Performance and OEM Parts, car parts
Part #: 102539005-57
KO Price: $84.06
Shipping: $0.00 - FREE Shipping! - & *Ship Group
Stock Status: In Stock - Ready to Ship!
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Book #: 10 2539 005
Description: Driveshaft Flex Disc
Applications: Carrera 4 99 thru 05
Manufacturer: O.E.M."