For a vertical spinner, it's been a few years since I was on the featherweight circuit but generally you want to be looking to keep your bulkheads and armour separate. One good knock to the side of the robot and that can be curtains for your weapon too.

As to material, I wouldn't recommend HDPE for structural members. If you want to stick with plastics then nylon would be the way to go. Metal wise, aluminium would be the recommendation.

Do you have any images of your design to share?