@norbauer does SprayMax make a satin? I am unfamiliar with their stuff... there is a matte made by them but it isn't listed as "2k"
If you want very matte, I have used Testors Dullcote on both Krylon and Dupli-color, and it always cures well, but as with many matte coats it does tend to lighten the color a bit. I'm not sure how much protection it affords to the finish as it goes on very thin (seems to be just enough to make the matting agent stick).
I will say that the Dupli-color clear (in the small "Perfect Match" can is not overly glossy, especially if you just do a couple of thin coats. The paint will definitely be glossier than without, but it's difficult to get a "wet" look with it which is sort of a benefit in this situation.
For satin over Krylon enamels, the Krylon Satin Crystal Clear (51313 in the older can, NOT the "MAXX") has been working for me recently after some trouble in the past. I'm sure there are better ones out there. Haven't tried it on auto paint recently but will do a test soon...