I have an Arvin manifold heater that includes a hot air pipe for the carb intake. A former owner has already installed the "trap door" in the floorboard. My problem is I would like to order a rubber mat for the car but don't want to cut a hole in the mat for the heater. If one did cut the mat , I suppose the right thing to do would be to put the outlet on top of the mat. But, if I do that it would have to be unscrewed every time the floorboards are removed to adjust bands. Decisions, decisions.
A round piece of steel exhaust flex pipe would make the connection between the heater on the manifold and the outlet on the floor. The flex pipe would/could be a slip fit in to the outlet. Your best bet would to be to cut a larger opening in the mat so it could be removed with out unscrewing anything. A friend has the outlet in the floor/toe board in his Model A PU, it melted his wife's plastic knitting bag.
Actually, the heater I have doesn't need the flex connector. The sheetmetal cover that goes over the manifold connects directly to the outlet in the floor. If I cut a hole in the mat I think I will start by punching round holes, probably 1/2",in the corners of the "square" hole so the mat will be less likely to tear at the corners.
That sounds like a good plan. You could also glue a fabric reinforcement on the underside.