I got my hands on a Chinon-357A from e-bay and, drum roll...
It fits! The eject button and the floppy slot align perfectly with the A1200 cutouts. After pulling off the faceplate, I just had to crimp back two tabs on top of the drive housing to get the case to close.
The only issue I have is that the eject button is a litte short and barely protrudes (just a few mm's). This is a slight problem since the button can pop-off if is pushed fully inside the case (the button relies on the faceplate to keep aligned). I wonder if I can find a longer replacement button?
I haven't tried to read or write a high density disk yet, but it seems to work fine with 880k disks.
-Bubba