opaque layer (makes sure the magnet holder doesn't shine through the bottom):
print normally
you can glue all parts together (not absolutely necessary if it fits together tightly) but don't glue the magnets themselves!
If you're unsure about whether the insert will fit properly: Print the test files in vase mode with an external perimeter/outer wall of 0.8mm. Check the tightness of the insert. If it's too tight, reduce perimeter/wall size of the shell until it fits your needs..