First off before any glue process begins the plastic should be washed off with soap and water then with Alcohol. This will remove any chance of a release agent clinging to the plastic. Then the area any glue will touch should be sanded on both sides. What I have used in the past and is still holding today after over 12 years is Araldite epoxy structural adhesive. It is also known as Fastweld #10. This adhesive does hold to plastic.