I second the recommendation to use Acetone or MEK on case plastics. It effectively dissolves the plastics together and can form a pretty solid bond.
I've used Acetone in combination with Epoxy resin filler to repair cracked and chipped cases. I then airbrush over the results to blend the repair section with the color of the plastic.
Below is an example of doing some of these repairs on an Epson Equity III+ case that had really fragile struts on the front case panel. I even filled in the struts between the drives bays with epoxy to add physical strength and keep them from further cracking.
Epson Equity III+ plastic repair 01.jpg
Epson Equity III+ plastic repair 02.jpg
Epson Equity III+ plastic repair 03.jpg
Epson Equity III+ plastic repair 04.jpg