Neat idea on the velocity stack. I tend to get tunnel vision and over look materials, like plastic. PVC when properly glued fuses the material into one piece. A piece of PVC pipe glued to a pvc washer would probably work out great.

I have bene making mine form copper water pipe and aluminum tube from tent poles and lawn furniture. First I anneal the tubing with a propane torch. I then chuck it in my lathe. I then set up a fulcrum with the tool post. Grease up the inside of the tube and "spin" it. I use the lathe jaw chuck key as my spinning tool. It has a handle made out of 3/8" material with a nice smooth radius on the end. Spinning is really easy. It is no trouble to work the material out to 90-degrees and up against the chuck jaws. You end up with a top hat shape.