I used pre-bent electrical conduit which fit perfectly over the external diameter of the heater outlet. This was just above my drain valve so 2 wide sweep 90's and it was right on my drain outlet.
As for CO, the exhaust from the heater was under a window, which probably leaked more air than my under belly. Also my CO detector was inside and very close to any potential CO infiltration.
This was slightly effective until temps went well below 0 NYE a few years back, and my old rig had exposed black and grey tanks, so after spending a few hours under with a heat gun I winterized for the remainder of the trip.
New rig has heated underbelly, so no longer a worry.
