I tried the Garmin route earlier today and was told they wouldn't help because I wasn't a dealer.... I saw a post on an archived form where Garmin jumped in and offered that advice but it didn't work for me.
I had to get my GNX375 manual from my Garmin dealer. It was not available on line from the Garmin download page. It does not contain a .stp file; just rough dimensions. I don't think there is enough there to make a CAD model. JMHO