There's nothing mystical about piston tanks- just a big syringe, as you take on water the air compresses behind the piston, when you empty the tank, the water is pushed out, and the air pressure normalises.
Mike's subworks does endcaps up to 4"
Here in the UK, there are people who do cast endcaps upto 11 cm, which is over 4"-http://marksmodelbits.ieasysite.com/mmb_site_047.htm
You can get Lexan tubing from McMaster Carr in the States. In the UK, Lexan tubing is much more expensive than in the States, so we tend to use PMMA/Acrylic or PVC pipe. The latter is cheap (drain or waste pipe), and just as tough as Lexan, but is usually filled, so opaque. As a lot of people prefer a see-through dive module acrylic cylinders tend to be more popular. Acrylic is a lot more brittle than lexan or PVC, but is more rigid and less easily scuffed, and strong enough provided you don't go dropping it on hard surfaces etc. Tubing used here in the UK will be metric based, whilst U.S will be imperial.