That is not a problem. PVC pipe comes in DWV - that is thinner than the standard Schedule 40 plumbing pipe. Hopot will have sizes up to 4". A wholesale plumbing supply house (anyone can buy at these places) will have all of the sizes, though once you get above 6", they start to get heavy, expensive and even worse, slippery.The Dan just gave me a great idea to try - make a tube out of pvc pipe to stuff it in.
I'd just have to find a big enough diameter to work ...
Your best bet is to have a local seamstress or upholstery shop make you a custom bag out of treated cordura. This stuff will not remain waterproof forever, especially on seams. You can push a plastic bag into the stuff sack and your sleeping bag into the plastic. The only thing that makes a compression sack different than a stuff sack is straps with buckles on the outside.