How do you glue these back on?

Fit the plastic pieces together dry, and using an acid brush, dab a bit of methyl ethyl keytone you can get from lowes into the crack. It will suck right in with capillary action right into the joint. It acts as a solvent adhesive and slightly melts the plastic as the glue allowing the original material to re melt and become the glue joint. Let it dry overnight before fooling with it. It requires the pieces to fit tight to work.