Bakster
I started the turret housing. Like the tubing, I searched around for something in the dimensions I would like that I can modify. No luck. I am sure if I looked long enough I would find something, but time is a premium for me. So, I am scratchbuilding it from styrene. I wasn't sure how it would turn out but I think I have a chance it will work
Bottle caps offer endless opportunities. "Blanking" caps (often polyvinyl) are another cool source.
Oh, and two words: Lug nuts. Your local AutoZone is chock full of cool stuff (often inexpensive) wanting a new purpose.