I had made a Genturi with PVC (similar to the commercial branded). The PVC did soften a little bit (enough to stick together when hot), and that's with cool air coming in from the bottom right next to the exhaust, you'll probably see the PVC distort with a long run as cool air may not enter freely being horizontal.
If you want to extend forward, I would use EMT conduit or similar exhaust piping with an adapter (use as big or larger pipe than the OEM exhaust) then after you turn 90 with the metal, then put your PVC genturi on. This will prevent too much softening of PVC due to lack of air-flow through the PVC.