I am looking for some help here. I found exactly what I need but in a different size and cannot seem to locate one. I have a one story home and upgraded the LP gas furnace last year to a 93% high efficiency downflow. In the process I fabricated a custom plenum with the idea in the future I may want to add a radiator (outdoor boiler) and A/C coil. By odd circumstance I ran across a Hot Blast wood add-on I could pickup at a decent price. I have the warm air piped into the plenum the gas furnace is sitting on and now my gas furnace is pretty much a chimney. The installation instructions state the upper compartment where the board resides is not supposed to exceed 100F. I have twin blowers on the wood add-on and it appears they will provide enough CFM to distribute a nice even level of heat in a stand alone operation. So I am looking for a backdraft damper I can install in my plenum so the wood heat does not rise up through my gas furnace and ruin the control board. I need a simple backdraft damper like what I understand to have been used in mobile home applications where the A/C system was self-contained. The plenum damper was louvered and would be normally shut, opening only by way of the gas furnace blower pressure. I have found one of these online but the dimension is only 12"x12". I need something larger. My return on my gas furnace is approximately 20"x20". Does anyone know where I can locate a plenum backdraft damper like the ones used in a downflow application where another self-contained system introduced air which you do not want to travel up through the furnace? Thanks.