Try Lexel— it has more structural flexibility than epoxy and won’t crack over time on HDPE or other bendable plastics (lots of kayakers, including myself, use it for modifications rather than epoxy on HDPE)
Whatever you try, first clean the plastic areas you want to bond with rubbing alcohol (to remove any oil/grease residue from manufacturing/packaging) Then roughen up the area with some 120-200 grit sandpaper. Clean with rubbing alcohol again. Apply adhesive.
You will have plenty of Lexel left over, but it’s a wonderful adhesive sealant to work with and will be useful on other projects.