don't waste your time w/ PVC.....PVC will melt - i tried to DIY my own GenTuri and couldn't do it, every bit of PVC i tried melted, no matter how large the air gap.
Electrical conduit (bent pipe) with a union works for the exhaust tip connection but again a PVC stack will melt.
Heres pictures of my failed attempt (i tried several different adjustments) and it would never sustain the heat produced from the exhaust of our Diesel fired Onan QD10(kw)
119 bucks for a GenTuri is money well spent!
Amazon - GenTuri