It's always hard to know what to suggest without seeing detail plans of your project, as to my own experience of HDPE Seraph is made almost entirely of HDPE it has been hit with MD's hammer and spent a while in the pit with Little Hitter doing it's best to knock a hole in it. Neither did any real damage,

I would not advise tapping into any plastic, this has been a problem even on THZ, use angle brackets, Blind hole captive nut or bamboo bolts (this is a stud (threaded) bar that goes through the whole robot with a nut on each end that holds everything under compression.)