Thanks - I found a couple, most require you to heat the HDPE before applying the adhesive. I also found PPX5 which is quite expensive but seems to be the best option , you just clean, mix and apply, working time is 2-4 minutes and ready in 90 mins.
I’m thinking that maybe I can drill holes through each side, then counter sink on both sides. Put Epoxy in the screw hole and hand screw the screw through, filling the inside with extra epoxy, then when cured cut off the excess epoxy and screw sticking out. Thinking it might create a rivet that can’t pull back through.
Probably over thinking this!!