The disk is designed to snap into the inner ring.  I would print it separately from the glass and then cut a small slit at the top to allow me to insert the glass and then insert the glass and disk together into the rings.  

The rings could be printed all at once, but I required two small helper disks (.2mm high) under the knobs on each side.  

I was also able to print the glass and disk together vertically, but it was not easy.