Just noticed another issue: Mounting a pulley to the bell of a motor is possible but not recommended for several reasons. The internal motor bearings are quite small and not intended to take side loads that the belt tension will put on them; I have had bearings literally explode from this problem. The belt tension will also strain the mounting screws and the further away the pulley is from the mounts, the more leverage it has. The motor base is relatively soft aluminium and the screws a small, so the threads will easily rip out of the base - again, I have several dead motors with this problem. Finally, the end bell can flex a tiny amount, causing the magnet to work loose. I had that happen to an expensive German motor, not a cheapo Chinese one.

I still can't visualise the shape of your design, but it has room for a 300 x 240mm cylindrical volume to fit the weapon. If you changed to a conventional beater and put the motor on the end, it will fit in the same space and all the tricky engineering problems will go away and costs will come down.

Back to your first post, one of the best fasteners for HDPE is the barrel bolt.